What is the number of petals in a flower?

The number of petals depends on the flower species. You'd need to be more ... er ... specific about what kind of flower you meant for us to answer the question. If you're just looking for a rough answer, "between 3 and 6" is a fairly good guess. Eudicots (not a species, but a large group) generally have 4 or 5 petals and monocots (another lage group) tend to have either 3 or 6 petals. (There are many exceptions to this rule, though, so if you want a definitive answer you need to specify the kind of flower you mean. Also, be aware that a lot of things you think of as "petals" may not technically be petals; the bright red part of poinsettia plants, for example, is actually a kind of leaf called a "bract".)