I say first come first served ('cause normally that's how I do it)
now after I voted, I feel like I should take back my vote. 50 commission offers are a lot. And if you get that ammount at ONCE, I'd say go with what you want to draw most!! The work would come out so much better, and if you finish them faster, hey, you might have more slots to open up!
Yeah, I'd say first ten offers I'd say as a general rule. I think it's good to stick to a method so people can rely on and trust you to work a certain way, but the occasionally acceptation may also be needed (Example being that you need to make money quickly for something or don't feel up to forcing yourself into certain works because you're under the weather or something...)
But really, as long as you mention in your commishes ahead of time that you have special rules for a certain set of them, I don't think many people will have a problem with it (Unless acting a bit childish...).
I think the best ones should be picked, because the first 10 is just a matter of commenting the fastest, and people with really good idea's might take longer posting and/or explaining them correctly...
I personally think it should be the favorite ones, lots of artists sometimes burn themselves out drawing stuff they really don't want to. Perhaps making one or two of them the most lucrative, at the end though I think it's the artists discretion for which commissions will be picked