I think Nintendo can afford to go bigger on numbers with GB/GBC simply because there are a minimum of 70 games that can come to the service; with third parties, it's easy to see that being closer to 100, perhaps more. SNES started with 20 games and NES with 30, so I see that as a ballpark for GB, given the size of the library rather than N64 (N64 might eventually reach 40 plus total, if we're lucky).
I also think Pokemon games will come to NSO this time. It's one reason why I think GB/GBC and GBA will all end up on the Expansion Pack rather than the basic subscription. Now, whenever GBA launches, I do think that'll be a smaller launch library (10 to 15 games).