How many Clow Cards does Sakura need to capture in the original Cardcaptor Sakura manga?

How to get close to this number

Reason from the length of the work. The original manga ran a small number of volumes, and a card-collection story needs enough cards to sustain episodes without exhausting the format. A dozen or two is what a short serialised run can support; a hundred would need far more chapters than the manga had.

The trap is that almost everyone knows this story through the anime, and the anime has a completely different number. Television adaptations of collection-based stories almost always inflate the count, because a weekly series needs far more episodes than the source has chapters. If your guess was much higher, you were remembering the adaptation rather than the original.

That gap is the reason the question specifies the manga. Fictional counts are properties of a specific version of a work, not of the fiction in general, and adaptations routinely add, merge or drop elements. Reading which version a question names is as much a part of answering it as knowing the story.