A: As noted above, most rescues are all volunteers and instead of having a shelter with lots of room, most rescues have volunteer foster homes which are people that are willing to take in a pet or two  into thier home until a forever family to adopts them.  Since most rescues are no-kill, how many pets a rescue can take in is dependent on how many volunteer foster homes they have and how often pets get adopted.   Even if a rescue had a facility similar to a shelter where they can house more pets, the fact that they do not euthanize limits how many pets they can take in.   Unfortunately, the supply of pets that are either unwanted or that their owners can no longer keep is much, much larger than the demand to adopt pets, so a rescue being "full" is all too common.