8-10 legs in a standard oven
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How long is a piece of string? There's no absolute answer, especially without knowing the weight of the legs, what kind of vessel you're using, what temperature you want to braise at or any recipe that you want to follow.
However, my normal braising practices mean that I'd give them about 4 hours at around 160f, checking for doneness after about 2 hours, with the expectation that I'd be reheating the legs when it was time to serve them rather than trying to get everything to be ready at the same time. I'd also parcel out the legs into several braising vessels, rather than try to pack them all together.