How many "not recieved" can you have?

Each 'not received' reduces the amount of giveaways you can create by one, so the amount you can have is equal to how many giveaways you can create (which goes up as you get more 'received's, so the number can change).

Nearly endless/without limit.

Not limited. Only drawback is it reduces free giveaways, so if you have ONLY not received - you will run out of slots sooner or later, and will be unable to make new giveaways. Sometimes it happens that someone registers account just to troll people, they do 3 fake giveaways and then it's no more, since at the start one has only three slots.
While "not received" is normal and not bannable per se, you still can get in trouble if you don't deliver the game promised and winner reports you. If your key happened to be duplicate, or region locked - you are safe, since you legitimately tried to deliver. If you just refused to give key to the winner for some personal reason (for example, you don't like winner's avatar) - you will be suspended.

Cheers everyone