How does the enhancement process in SuiteIdeas work? Is there a set number of votes needed before they take a piece of work onboard?

No, it's much more arbitrary than that. I think it's more like they'll use the number of votes as a factor in their decision making. It's a bit of a black-box, but IMHO whether the enhancement is likely to have revenue/marketing impact and how easy it is for their developers to do plays a much bigger role in which ones get selected.