I like 100-500 little things (I didn't mind the 500 bananas in DK64), 5-15 big things (25 golden bananas in DK64 was a bit too much for me), and 2-3 secret things per level.
I also don't see why we need to be restrained to the same number of collectibles per level. Why can't we have a smaller level with 50 Pagies and a larger level with 150 Pagies? I wouldn't have too much variation, but I think some variation would actually help to balance collecting in different levels. I feel like we could have had 50 notes in Mumbo's Mountain and 150 in Click Clock Wood. Of course we need round numbers and we can't have a level with 134 Pagies.