if you show up at FW kennel and the dog side it is fully (only happened twice). (the cat side never fills up)
they will call the other kennels and see where you can go... or if you dog is small and they don't have any cats - you may elect to go in the cat room.
there are 5 kennels - they don't fill up easily....
TTC (MK), FW, Epcot, Studios and AK.
the TTC and Epcot are the biggest. I have never know them to be full.
fw can hold around 40 dogs and 8 cats cages. (might be more). this is the smallest kennel on WDW property.