Have a 3 month old baby, formula feed & has had 15 dirty diaper between 2 days. Is that normal?

I assume. you mean by "dirty diaper" diapers containing stool. 7 or 8 stools per day may not be unusual, but if it is a big change from previously, you should take your baby to her pediatrician. If she is making fewer urine soaked diapers, it could mean she's dehydrated and needs prompt evaluation.