I am wondering exactly how many cards you must have in your card pool before you may activate Relentless. According to the rules a cards is moved to the card pool after it is played. So would that mean that you have to activate relentless before the card is on your card pool? If so does that mean I must have 2 weapon cards in the card pool first before I can activate it meaning I could only use it on a 3rd weapon card?

The card goes to your card pool once it is played, so before the response window for "after played".

So then it would work after I play the second weapon card into the card pool?

Yeah after you've passed the check to drop yer second weapon card into the card pool, you can use Relentless