The phrase "unbelievable number" refers to a quantity or amount that is very surprising or hard to believe. It suggests that the number is so large or so impressive that it may be difficult to comprehend or accept as true. Full definition
It's unbelievable the number of spelling, grammar, and punctuation mistakes I see in resumes. (thebalance.com)
Night dives at Manuelita promise close encounters with unbelievable numbers of whitetip reef sharks as they hunt their prey. (villascostarica.com)
Shallow (2 - 6 feet) and with a whiter than normal sand bottom, these tarpon feeding grounds stretch for over fifty miles and provide the dining room for almost unbelievable numbers of tarpon. (ambergriscaye.com)