Two times in a row, the grocery cashier said nothing when someone in the express line had more than 15 items. Should I complain?

There is no law

Besides what looks like more than 15 items may actually be less...For example say canned corn is on sale 3 cans for $1.50, those 3 cans only count as 1 item

Most stores have a policy against the cashier saying that to the customer.  Sorry.  It also isn't a law karen.

It's a private company policy, not a law. Private companies can pick or choose how they want to enforce their own policies.

No one can blame the cashier for not wanting to risk violence from some lunatic. People are loony these days and even a union checker isn't earning enough to take on the kind of stupid that's roaming the streets right now. 

Stop counting other peoples items. Cashiers can exercise a degree of flexibility. 

"Hey kids, get off my lawn".  Don't be that guy.

? -   You need more to do in life.....  

In my neck of the woods the sign says "about 15 items".    The store employees are not police and they get tired of hearing you whiners complain. 

Sorry, but it would take more time for the checker to count someone's items and then tell them sorry, but you have 16 so you have to load up all your stuff and go to a different line.   Not going to happen so save your breath.

"then that rule shall stand"    How the store chooses to enforce (or not) suggested courtesies is not your decision.