Yan,

       It depends on the process that is used to release the structure. for
instance, PolyMUMPs process of MEMSCAP Inc., gives the etch hole dimension along
with its pitch in its design rules data, so that you could decide on the hole
density for ur structure based on its dimensions.
So, its the process people, who'l be specifying on these details based on their
experimentations, else u'l have to try and establish your own in case of any
custom requirements.
Also, like matthew suggests, u may contct the company itself for more
details....

Swamy M
IIT-Kharagpur