Here's one opinion, which appears to be typical:
"The only equipment that you're going to need for P90X is a set of free weights (all ranges, in my workouts I've used everything from 5 pound dumbbells to 25 pounds) and a pull-up bar. Now, either and both of these can be replaced with a set of Power Bands. There are other odds and ends that you are encouraged to use, such as a heart rate monitor and a yoga mat, but you don't need them." (Perspectives forum thread)
When in doubt, you can contact the company, to see what they say you need or might want for P90X. (Beachbody customer service)
