Could you make a countertop out of plywood and paint with floor paint?

Hello Carol! You sure could do that if that's what work for you. Doesn't hurt to try anything with the idea that if it doesn't work you need to move to Plan B. I would add few coats of marine spar varnish that is water proof. It is my understanding that the marine spar would be food safe after it has cured. Curing takes 30 days. It will feel dry before it is cured--curing is the the place where the spar varnish reaches a maximum hardness-different than just feeling dry. The Hometalk article I am including says that floor paint is food safe but I am not so sure about that. You might have some moisture issues around the sink. https://www.hometalk.com/diy/kitchen-dining/kitchen-dining-furniture/new-use-for-floor-paint-to-revamp-a-kitchen-32109133?expand_all_questions=1

New Use For Floor Paint To Revamp a Kitchen

Don't see why not. Any paint will work with a sealer coat. Some may last long time some may not.

https://www.hometalk.com/search/posts?filter=paint%20wood%20countertop

What dimensions (length and width) of a countertop are you wanting?

Gk has a good point regarding the water impermeability of painted plywood. If you are intending to use the built countertop in a kitchen, it will need to be a surface that can be frequently cleaned and sanitized to make it food-safe.

Price out all the materials (lumber, tools, hardware) for a diy project.

It may be more cost effective to purchase a ready-made piece of laminate.

If you have an IKEA near you you might want to check their countertops. Here is one, 98" long for $62.

https://www.ikea.com/us/en/catalog/products/90276612/