There is absolutely no need to have 15-20 tricks for street performing. If I had to sit through 15-20 tricks, much less, only cards and coins, I'd be searching desperately for an excuse to leave. Now, There's no reason that you shouldn't at some point have 15-20 tricks at your disposal. But it's unwise to "focus" on that many at a time. Technically if you were performing on the street, you could take one trick, just one, have it down perfectly, and do it 20 different times in a day, and it'd be original to the audience each time. you might of course want to mix it up for yourself, so maybe having just one trick isn't the best idea, but it's to put things in perspective. Why limit yourself to cards and coins? Be prepared for all situations. Business cards, Paper Money, Cups, Balls, Walnuts, Leaves, Pencils, Rubber Bands... Spontaneity is beautiful. When choosing these 15-20 tricks (which I suggest breaking into sections when learning them, meaning, learn them in groups of... 4-5 maybe?). Have a good variety of different plots, too, have some packet tricks, different audience participation ones, solo work, etc... Be ready.
Art Of Astonishment is A suggestion of mine.