One seed, many wallets…

I am just trying to keep track of my various not well organized tokens.
It would be helpful to know:
-Can there be multiple seed phrases for a given mm browser extension at the same time? Or only one at a time?
-Will the meta-mask browser extension always show me every eth wallet address that is derived from the seed or seeds? Or is it possible that some might be hidden from my view? If yes then how can I be sure to see all of them.