updating count fields

I have to build a database of firms that can have more than one address. It is formed by two Tables:

Table1 Firms

Fields

FirmID Firm name Number_of_sites

Table2 Addresses

SiteID FirmID SiteAddress

Do you know if I can update the Number_of_sites field by using an UPDATE query with a count on the fields of the second table joined to the ones of the first?

update Firm
set Number_of_sites = (select count(*)
from Addresses
where FirmID = Firm.FirmID)