I took the advice and went with 4 onions. Worked well, though I could see even going with 5. The bread crumb bottom really ties it together. Great side dish. Would be good a brunch too.
Two eggs are not enough. I used three and it was still DRY (very tasty, but dry). Five or six would be much better - with two it was more like a pile of onions than an onion pie.