I usually play at least two or three games and very rarely just "lock" myself to one unless I'm very focused on it, although that does happen from time to time.
When I'm "game-locked" to just one title it's usually because it was just released and I'm drawn in until I complete it. In recent years games like the Mass Effect trilogy pretty much "locked" me for a few days playing only that (a good five days or so for my first ME1 play-through, around a week for my first ME2 play-through, and maybe 3 days or so for ME3). That, or games like STALKER SoC (couldn't play anything else for a good week, maybe more), Dragon Age Origins (first play-through took me a good 4 or 5 days, maybe more, but the point is I couldn't and didn't want to play any other games), heck even Dragon Age 2 (first time playing it I took a good 3 days without touching anything else before finishing it).
Those are just a few examples, there's many more (and much older ones of course). The gist of it is that I'm only focused on one game if I'm planning to complete it and it's new, and I like it (duh). After I complete them, however, I usually have a good 3+ games to switch between on a regular basis. As of now I got about 300+ games and maybe half of them are installed, and out of those installed I've been playing about 4 of them lately. I just started a new game in both Dragon Age Origins and DA2 (in preparation for Inquisition, although I'm doing this in advance and not just a week before Inquisition's release) but I'm also playing my occasional skirmish in Tiberian Sun, Soulstorm (WH40K), playing my daily ME3 multi-player (at least two Gold matches per day) and just playing Primal Carnage for the heck of it from time to time (and Borderlands 2 as well).