Tipically a min of 20% down is needed for most coop's.... Just keep in mind that not all coops are the same and there are coop's in NYC that will not let you take out a motgage on the property and require 100% down.
It really depends on the co-op. It's important that you work with a Realtor, who can help you determine what your options are. I notice you asked another questions about co-ops and have included a link to help you become more informed. Please feel free to contact me with any additional questions. I will try and help you or refer you to someone who can.