How many birthmarks can one person have?

It's possible to have many "birthmarks". Much depends on the definition of birthmark. For example, most people have some "moles" on their skin, but unless present at birth these are not really "birthmarks". "Classic" birthmarks (ie brown, black, red/ pink or blue/ grey patches present at birth) may occur as "multiples" - sometimes a larger mark may have smaller "satellites" nearby. The "Cafe au lait" mark can occur as a multiple - if someone has 5 or more such marks this MAY indicate certain neurological conditions.