I forget who it was...could have been the Professor. When asked how many card tricks he knew, he paused for a moment and then answered, "about eight."
Remember, it's not quantity, it's quality. No offense, but I am willing to bet you couldn't perform 20 of those tricks right now flawlessly. If you can, good for you.
I don't consider reading or watching a trick once or twice "knowing it." Tricks you know are the ones you perform and practice non-stop...the ones you didn't read or watch, but truly studied.
Oh, and to answer your question. About 10.
Well said, I totally agree. Sure you might know a ton of tricks but wouldn't you rather know only a few but have them down perfectly? I mean look at David Blaine. He performs the two card monte often but he does it perfectly. You can't perform 30 tricks PERFECTLY. It just isn't going to happen. Thats just my opinion.