Those boards with population numbers on

So was watching a daft Carry on Cowboy film....the baddie rides into town - theres a man painting the number 404 onto a town population sign ......theres some shooting and of course the painter has to change the number to 401 - thing is .........

what is the history behind those 'population' boards and are they only common in US ?

Great question. I hope a better U.S. historian answers it for you.
There was a town I lived near in Virginia that had a sign that read: "Population 1,001 and 3 old farts*" (I kid you NOT!) And the sign never changed. It always made me wonder what you are now wondering too LOLOL

*number is a guess but the 3 old farts is 100% my memory!

We have those signs in Canada... don't know if they're really widespread here but I do know of many places that have them. They update them every few years as far as I know.

My mom lives in a tow with a sign that reads "population 36,000". According to the last census, the population is now over 150,000, but I guess they don't have the funds for new signs.