I'm now up to 8 lenses (including the one out on loan to Dennis Sulz):
45mm f/2.8 C and 45mm f/2.8 N, 55mm f/2.8 N, 70mm L/S, 80mm f/2.8 S, 110mm f/2.8 S, 150mm f/3.5 and 210mm f/4.0.
I find them all to be of excellent quality, and individually would recommend them all. I don't need them all, and should get rid of at least one of the 45mm lenses, but here are some of the reasons I have as many as I have:
1) a whole bunch of them were bought for silly low prices on eBay;
2) when shooting hand-held, I really prefer using lenses that both take the 58mm filters and can be used with the Mamiya focus assist lever - the 55mm, 80mm, 110mm, 150mm and 210mm;
3) the 70mm L/S lens has a leaf shutter, and is very useful for group shots (e.g. weddings, using fill flash);
4) I bought the 45mm C lens from a store - I got to see it and hold it and buy it with a guarantee!;
5) I bought the 45mm N lens because it is smaller, and newer;
6) 55mm, 110mm, 210mm is a great trio; and
7) 45mm, 70mm (or 80mm), 150mm is another great trio.
It has always been my understanding that the 150mm f/3.5 is at least slightly better than the 150mm f/4.0. I've never been much interested in the 150mm f/2.8, because it won't work with the focus assist lever.
From this, you can probably come to a couple of conclusions:
1) I have trouble saying no to good lenses, for cheap prices;
2) ergonomics mean more to me than large apertures; and
3) I tend to think in terms of sets of lenses, rather than individual lenses, and I find that sets of three work well for me (for 35mm, my usual kit is 24mm f/2.8, 35mm f/2.0, 85mm f/2.0 - all Olympus Zuiko).
The N series lenses, being newer, are a good idea, but I don't think there ever was a 110mm lens in the N series, and there were very few 210mm N series lenses made before that length was discontinued.
Hope this gives you some useful food for thought.