Here's my interpretation of a travel, please correct me if I'm wrong.
I've heard people say you're allowed 3 steps between catch and throw while running and it's not a travel. The rules (XV.D) say the disc must be released BEFORE the third ground contact. The way I figure, that means 0-2 steps.
1. If you catch the disc with both feet on the ground, that's 2 ground contacts. No further steps are allowed.
2. If you catch with one foot on the ground, you can take 1 step.
3. If you catch with both feet in the air, you can take 2 steps.