[Suzu]

Personally, I think Rider series with fewer Riders are fine the way they are. The more Riders you introduce, the harder it is to make all of them equally good or interesting characters. Of course, not all of them can be at the exact same level of character development, but it should be more than: Hello, here's another rider, watch how he's cool in one episode and never does anythin useful again.
There's multi-rider shows that do good (Gaim), others that are... not quite as good (Kabuto) and some might be terrible. As long as a writer does a good job on having so many characters it's great, but a small cast of Riders is fine with me as well. I like casts that have more non-Rider characters because it makes the Riders seem much mor unique and it's easier to keep an overview. Except for Gaim, most of my favorite Rider seasons just have a few Riders (OOO, W, Blade...) and I wouldn't need any more riders in those.