Apple fixes new zero-day used in attacks against iPhones, Macs

Is there a trusted process on ios / macos to determine if you were attacked by these recent 0days? I hadn't upgraded to cover the last batch of 0days for a bit, and then safari crashed a few times in a row and one morning my open browser was on an full-page ad that seemed a bit suspicious as to how it ended up there.. I'm assuming web searching and installing some third party tool will lead to more trouble than a fix..

Is there an information/complexity theory that can put bounds on the number of 0days? Does iOS have millions of tens of these?