For this kind of job, I would say that two coats are the bare minimum, and three would be ideal. The main purpose of a polyurethane coating is to seal out moisture and protect your cabinets from moisture damage. Of course, your cabinets are unlikely to see rain or other heavy water exposure, but you shouldn’t underestimate the destructive effects of humidity.
The first coat of polyurethane will act as a sealer that keeps all moisture away. The second coat might be described as a sealer for the sealer. In other words, it sandwiches the first layer in place so that it cannot be chipped or peeled away. Subsequent layers will only add more protection. Of course, this kind of finish will eventually wear away, but its longevity will depend mainly on how many coats you apply. At the same time, any more than four coats would be a little excessive. Your cabinets probably aren’t going to be seeing serious abuse, so there is certainly no need to go crazy here.
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