One member of the guild can choose or be distributed in one fray for a brawl and only one. Often, there are more members in the guild than the number of frays available.
There is the reverse too. Some frays are harder to fill for some guilds (especially gold foil ones), and they can be left unfilled. It's just a matter of optimizing the number of crowns/merits the guild wins, and if it's worth filling a fray or not.
See also Anytime Tournament.