When you say “She has a lot of charms”, the charms is a noun and you’re telling the listener that this female owns a lot of charms (like charms that go on jewelry). When you say “She has a lot of charm”, you are explaining the female’s personality to the listener. You are telling the listener that “she” has a charming personality. 🌈 Nicky

Charms = little collectable items that girls collect for, say, a bracelet. Charm = has to do with personality.

There is also a cereal breakfast called "Lucky Charms"

I interpreted your meaning differently from the other commenters. These phrases can both be used to describe personality and basically mean the same thing. "... a lot of charms" is emphasizing that she has a lot of different qualities the speaker finds charming. "... a lot of charm" is emphasizing that the speaker thinks she is charming. If these sentences are being used to describe a person (as opposed to their belongings), one says that she is charismatic and one says that the speaker is drawn to multiple things about her which may or may not be charisma.