When Link finds a Rupoor in some games, he loses Rupees out of his wallet. How many? Is it the same every time? And in every game?
It depends on the game. You can find all you need to know here on the Zelda Wiki
The Rupoor (or Black Rupee) appears in several games, and has a slightly different effect in each.
Four Swords: Commonly known as a "Black Rupee", it causes a random amount of rupees to fly out of the wallet, which can then be picked up again, if you can manage. Note: This can actually be beneficial: if the Links are under the influence of Rupee Fever (all Links have full health), the amount dropped can be doubled if they are taken back.
Phantom Hourglass: This is the first time it is encountered as a "Rupoor" by name. In this game, you can find a 10 or 50 Rupoor, which just deducts the amount from your wallet. This cannot be retrieved.
Skyward Sword: Commonly found in the "Thrill Digger" minigame. In this case, when Link finds one, he loses Rupees. In the "Fun Fun Island" minigame, if he lands on a "Loser Space" (indicated by a Rupoor) he must give all his Rupees to Dodoh. He can also find them randomly, but has the chance to turn them into regular Rupees by sprinkling Glittering Spores on them. Each time he loses the Rupees, again, as in Phantom Hourlgass, these cannot be retrieved.