does anyone know how i can get iron nuggets ? or can i buy any off someone ?

Hi there you can hit the stones on your island with an axe and some will usually pop out. Sometimes just stones or clay will come out though. If you have run out of rocks you can use a nook miles ticket and fly to another island and hit some rocks with your axe there!

How many iron nuggets do you need?

Yeah, just hit rocks with an axe (or if you get a shovel a shovel will also work) and sometimes you'll get iron. Other resources from rocks include stone, clay, and if you're lucky, gold nuggets. (And at least one rock on your island per day eventually becomes a money rock)