PIC12F1822 counter uses 2 PPM crystal oscillator in Real Time Clock. The PIC sets the DS3231 module to output accurate 1 Hz which is used to gate the input frequency, I use this module because it is cheaper than other 2 PPM crystal oscillators. The 3V battery isn't needed.
The frequency is displayed on line 1 by 7 digits with leading zeros removed down to 3 digits. Signal input is a TTL type.
Period is displayed on line 2. The period is calculated. For frequencies greater than 1000Hz the display is ns and for less than 1000Hz the display is in us (micro sec).
OLED module is I2C 0.96" SSD1306. Contrast of oled can be set in software. Included C code and circuit diagram.

Color Oled display 0.95", 96 x 64, SSD1331 driver. Pic drives the Oled using SPI. The counting resolution is 1 Hz for frequencies up to 8 MHz. Included C code and circuit diagram.
The frequency is displayed on line 1 by 7 digits with leading zeros removed down to 1 digits. Signal input is a TTL type, 3.3V max. The time base is generated by the 8MHz crystal oscillator. The CPU frequency is 32MHz from the PLL.
2 diodes are used to reduce 5V to 3.5V for the PIC's Vdd, it is needed because the SSD1331 inputs don't tolerate 5V.
Period is displayed on line 2. The period is calculated. For frequencies greater than 1000Hz the display is ns and for less than 1000Hz the display is in us (micro sec).
OLED module: https://www.ebay.co.uk/itm/282379866279?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m2749.l2649

You are free to use the circuit diagram and the software with no limitations

Counter based on PIC16F1827 . Included C code and circuit diagram. Input is a TTL level. The counting repeats every about 1sec. The 8MHz crystal is for generating accurate time base for counting the input frequency. The LED module doesn't flicker, LED brightness can be set in the code, value can be 0-15. The 7 segment LED module is driven by 3 lines of the pic SPI.
TMR0 input can count frequency up to 50MHz if the signal is 50% duty cycle. If the duty cycle is 10% then the max frequency is 10MHz. TMR0 prescaler divides by 2 to increase the input frequency range to 16 MHz.
The counter measure the frequency for 0.1sec first, if the frequency is greater than 2MHz TMR0 prescaler divides by 2 to increase the input frequency range to 16MHz with resolution of 2Hz, if the frequency below 2MHz TMR0 is set for no prescaling and the measurment is with resolution of 1Hz.

Software includes PIC code. You are free to use the circuit diagram and the software with no limitations.

Counter based on PIC16F628A . Included C code and circuit diagram. Input is a TTL level. The counting repeats every about 1 sec. The 20MHz crystal is for generating accurate time base for counting the input frequency. The LED module doesn't flicker, LED brightness can be set in the code, value can be 0-15.
PIC16F628A TMR0 input can count frequency up to 20MHz if the signal is 50% duty cycle. If the duty cycle is 10% then the max frequency is 10MHz.
The counter measure the frequency for 0.1sec first, if the frequency is greater than 2MHz TMR0 prescaler divides by 4 to increase the input frequency range to 20MHz with resolution of 4Hz, if the frequency below 2MHz TMR0 is set for no prescaling and the measurment is with resolution of 1Hz.
I tested the counter by connecting the frequency input to the 20MHz crystal, it read 20000422 instead of 20000000. The error is due to time of instructions to the CPU to start and stop the count.
The 7 segment LED module is driven by 3 lines, the pic generates a software SPI.

Software includes PIC code. You are free to use the circuit diagram and the software with no limitations.

Counter based on Arduino Uno or Nano. Included INO file and circuit diagram. Input is a TTL level. The counting repeats every about 1 sec. The LED module doesn't flicker, LED brightness can be set in the code, value can be 1-15.
The Arduino generates an accurate 1 second time base for the counter by cascading timer0 and timer2. The link between digital inputs 3 and 4 connects the output of timer2, 250 Hz, to input of timer0. The software waits for the output of timer0 to go positive to start the count of frequency input to timer1. Timer1 is a 16 bits timer, it overflows at the count of 2 power of 16, that in turn advances over-flow register. At the end of the 1 second the 16 bit register is recorded.
Max7219 lib is used: https://github.com/An7orAhmed/MAX7219Segment
You are free to use the circuit diagram and the software with no limitations.

Counter based on PIC16F628A . Included C code and circuit diagram. Input is a TTL type. The counting repeats every about 5 sec. The 32K crystal is for generating accurate time base for counting the input frequency. The MCU uses the 4MHz internal oscillator.
PIC16F628A TMR0 input can count frequency up to 50 MHz if the signal is 50% duty cycle. If the duty cycle is 10% then the max frequency is 10MHz. TMR0 can count up to 2MHz , the prescaler is used to increase the input frequency range to 64MHz. Frequencies up to 1MHz are counted with resolution of 1Hz, frequencies above 1MHz are counted at resolution of 32Hz.
I tested the counter up to 2MHz only, I don't have the means to do better tests.
The 7 segment LED is common cathode, cc8 is the common cathode of the digit on the left. 100 ohm resistor for the segments is for a current of 20mA, the max output current of the pic is 25mA, so if you change the resistors make sure you don't exceed this current.
To drive 8 digits from 5 outputs I used a system of 2 out of 5 that gives up to 10 outputs by AND gating to outputs. I used one output to drive the base of the transistor and the other a diode. the emitter of the transistors are elevated by 0.7V by a diode, this to make sure they are switched off when the diode at the base is low. It is possible to reduce the number of digits by deleting them and by deleting the lines in the code that drives the digits.

Software includes PIC code. You are free to use the circuit diagram and the software with no limitations.

All transistors are BC337 or equivalent.
Total 5V supply current can reach145mA.
The 8 digits display is common cathode, for red LEDs the 7 resistors of 100 ohms enable current of 20 mA total, average of 2.5 mA through each segment which is on for eighth of the time. To increase the brightness the resistors can be decreased to max current of 25 mA. The seven segments, a to g, of one digit are linked to the corresponding seven segments in the other digits. cc8 is the common cathode of the digit on the left.
The 8 digits are multiplexed by 5 pic's outputs, for each group of 2 out of 5 a digit is switched on. The diode to ground from the emitters is to insure full switch off when the gating diode at the base is low.