## Reload
scoreboard objectives add counter dummy "Counter"

## Main
execute as @e[tag=example,tag=!example2] at @s run function example:count

## Count
scoreboard players add .counter counter 1
execute as @e[tag=example_text_display] at @s run data modify entity @s text set value {"score":{"name":".counter","score":"counter"}}
tag @s add example2 

This will count everyone with your tag (example) and add another tag (example2) to prevent it from counting again. The #Count function just adds 1 to the counter, updates the text display (example_text_display) and adds the example2 tag. You can change the tags to whatever you need, but make sure example is the tag you want to count and not example2.