"You can play cards by themselves if they are the same mystery as cards already on the table. For instance, if another player has a Giant Squid meld in play, you can play one or two Giant Squid card(s) in front of you."
If I have three (or more) of Giant Squid cards in my hand, can I play all of them in a single turn?
Yes. In games like this (rummy style), you need to have something melded in order to play. This version allows you to lay off of other people's melds. But you can play each card by itself (3x) or lay off all three.
The distinction they are making with "1 or 2" is that once you get to 3 cards, you are technically placing down your own meld and so don't need anyone else'e meld to lay off. But even at that, you could lay off 1 card, and then lay off 2 cards which is the exact same net effect.