If you want to add more than one layer mask to a single layer, you can put that layer into a group and add a mask to that group.
In fact, you can keep repeating this process of nesting layer groups and adding masks up to 10 times (5 times in CS4 and below). This will give you a total of 11 layer masks for a single layer!
You can also do this same process with vector masks or both if you wish.
Here’s an example of how the layers panel could look.