I think in an average league, two DL slots is just about right. Lets you have a slot if you have a star hurt long-term and another if you just need a fill-in while one of your regulars goes on the 15-day DL. I started a keeper league a few years ago, and I went with two slots.
That said, my main league is one I joined about five years ago which had been around for quite a few years before that (it pre-dated fantasy baseball sites, if that tells you anything). That league has no DL slots and requires a certain number of backups at each position (2 at each infield position, 5 OF, 5 SP, 4 RP).
It's actually a really fun and challenging league. It takes most new managers a year or two to understand how it works, but it's a unique challenge to deal with injuries when you have to either sit on a guy (and his salary) for a long-term injury or drop him outright. It's a part of what makes that league unique, and now that I'm commissioner of that league as well, I don't think I would entertain the notion of adding DL slots.
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