cash reserve

That depends.

Do you have an elevator? Elevators can be major money-suckers when they need to be repaired and/or replaced. How old are the roof and windows?

no elevator at all

i think the roof and windows are in good repair...

You "think" isn't good enough. Has your attorney done his or her due diligence? They'll be able to tell you how recently the roof and windows have been replaced. The lifespan of both is about 20 years. If the building is less than 5 years away from replacing either, they'd better have at least $300K in cash reserves right now. If they're looking at 15 years out, then $100K should be a good minimal cash reserve for a 10-unit co-op.

6 months maintenance.

I think it is very dependent upon the annual budget, what it provides for, how much is being collected monthly, overall age and condition of building, etc. A 100 year old place has different needs than a 20 year old place. I think 20 years for windows is a little short if they are quality windows, but whatever. Look also at the history of special assessments.

The question can't be answered with a formula. Personally, I am far far too risk averse to assume the financial risks of such a small building. One major thing and I'll be draining savings accounts in ways I hadn't planned for. Another shareholder defaults--or God forbid 2 default at once, facade is discovered to be crumbling beneath the pointing, etc. I can't deal with those kinds of bills. I think the smaller the coop the more money you better have on hand just in case.

Shareholder default rates in co-ops have always been historically low, especially in buildings that require at least a 20% down payment and six months' mortgage and maintenance in the bank at closing.