If you forget to negotiate with them, and you go with O'Reilly, they are really good at sending free books for book signings to conferences which can be a really great chance to both get the word out and meet people that care about the tech you wrote about.
When book sales are split between multiple parties I can't imagine it's okay to get a bunch of free ones and sell them yourself, keeping 100% of the proceeds.
What's a realistic order-of-magnitude number there? 50? And do you have to agree not to sell them?