Regaling my list of plays this year yielded two that had 20 holes. European Club and Barnbougle Lost Farms. Sebonack has 19. Not sure off the top of my head.
I do think that having 19 holes is a great idea for whenever you need to take a hole out of commission for maintenance/etc. you have a backup so I really like that idea. 20 holes is probably overkill and know Pat Ruddy saw 20 good greensites and built 20 holes.
How many others have more than 18? I think there are quite a few in the UK.
Actually, I can't think of many in the UK, but most of the leading clubs in Australia have built a 19th hole somewhere on property for the maintenance / renovation angle. The first one I saw was Michael Clayton's for Kingston Heath, which they actually use for tournament play, skipping the little tenth. We have built one for Royal Melbourne, Royal Adelaide, Woodlands, and Yarra Yarra.
I built 19th holes for Sebonack and Stone Eagle and Streamsong, already mentioned, but those were strictly as extra tie-breaker holes near the clubhouse. We famously used the 19th at Streamsong for a sudden death tiebreaker to qualify for match play at the Renaissance Cup, when we had too big of a field there
I agree with you on 20-hole courses, though . . . it just seems too many to me. If you've got 15 holes, that's because there was only room for 15, but if you've got room for 20, you generally make the decision which ones you want to build.