2.8 U.S. Gallons (for the 2.5i, 3.0 maybe a touch more)

AT models generally have 0.2L greater coolant cap. than MT's.

Of course, if you are draining it, you are not able to get it ALL out of the block.
(So a single jug of the BMW blue coolant may be sufficient.)

 

thanks, I'm thinking about draining it and wandering how many gallons of BMW coolant I need.

 

You will probably find that you only get about 3/4 gal. out of the radiator (even with squeezing the hoses), and maybe 1 gallon out of the block if you are able to find the drain plug hidden behind an O2 sensor on the passenger side. That leaves at least a gallon still in there - don't make the mistake that others have of starting to fill with too much distilled water and not being able to bring the mixture up to 50/50.
Given the difficulty of getting everything out, short of performing a complete cooling system disassembly, or having the machine a dealership or indy uses, a good DIY approach is to do the (easy) partial draining every 2 years. That way, the coolant mix is kept in acceptably good condition over the long term.
Don't use flushing compounds because some will always remain in. Similarly, I think it better to not use a hose to flush tap water through. One other caution is to treat plastic radiator and bleeder plugs with due care - I see many threads were owners of other BMW models have attacked them inappropriately and suffered the consequences.