How many coats of polyurethane should I use on my kitchen table?

Three coats of a water based polyurethane should work. Oli based poly, varnish, has a yellow cast to it and will yellow over time.

With the primer and the paint there I would use 2 layers, but let it dry WELL between layers. 24 hrs should be minimum drying time, I would give it more.

I redid a table top recently and did 2 coats and let it dry almost a week before coats, I actually think I may put one more on for good measure

but mine wasn't painted it was stripped, then wood stained

I refinished a table that old shellac on it and I inquired at Ace Hardware that had an employee that has refinished wood furniture for years. This was a kitchen table that if you put a cold glass on you would get a white ring. The advice I got was if I wanted to be able to wipe this down with a sponge after meals and would get heavy wear seven coats with a light sanding between coats would give me the longevity I was looking for as I did not want to refinish this again any time soon. That was over ten years ago and still looks wonderful.

Water based only - polycrylic works great for these type of projects. Never use oil based poly products, it will discolor your paint.

When I did a brown bag decoupage on a large round table I put 6 coats of poly. I find if you want the glass look (super shiny) but it's all in ones preference

Usually three coats lightly sanded in between is good enough but if you have the time and energy another coat would give it more protection. Love what you have done already as it is a new idea for an old table, especially with the black..

You might try a resin glaze. It is really durable. It looks like a bar top when it's finished.