QWhat does "you can put up to 50 of this card into your deck" mean?

ANormally, you can have at most 3 copies of the same card in your main deck, but this rule doesn't apply to Onion Patch. You can put up to 50 copies of it into your main deck. Note that you can't put Onion Patch into your evolve deck, because it isn't an evolved card.

QDoes Bergent, Onion Patchmaster's "if an Onion Patch on your field would deal damage, it deals that much plus 1 instead" ability increase damage dealt by Onion Patch's "Strike - Select an enemy follower on the field and deal it 1 damage" ability as well?

AYes, it does.