The ideal answer to this would be as many as you like but in reality if you have 3 website you need to buy it 3 times, which sucks, but do understand the developer needs to make money to fund the updates.
Some plugins have no way of tracking so once installed the developer has no clue where its installed meaning you can install it on more, but that is a no-no.
What I am finding on CodeCanyon innocent people like yourself and myself ask this question all the time and it turns out the developer does not even have a clue and recommends you download the Extended License, but if you read the descriphtion of that:
Use, by you or one client, in a single end product which end users can be charged for. The total price includes the item price and a buyer fee.
I think CodeCanyon need to offer a third option for developers for an unlimited use of a plugin, costing what they feel fit.
Hope that has helped.