Do what they did on Windows, pick the 4 or 5 most popular. (In that case it was browsers)

I know, I know, what about the less popular search engines?

Well. It's better than Google takes all, and customers get easier access to 5 search engines instead of 1. (Yes they can already choose others. Almost no one bothers)

Maybe they could rotate the last 2 choices between a field of say, 20.

Thing is, if no one's heard of the option, I doubt they'll choose it, so there's not much point offering beyond say, 5 choices.

I'm guessing though.