One 8 inch mat under the beam, no deduction. There will be the harder regulation mats, but you can put an 8 incher on top of that under the beam. If you're mounting off a board or panel mat, that may not be placed on top of the 8 inch mat. But really the head judge is not going to take equipment deductions without a warning anyway. I suppose they can but I have never seen that happen. They always give a warning that you have to move X here or whatever. The only way you can go wrong is to choose to ignore the warning and take the deduction, but you can place an additional 8 inch mat over (minimum) regulation competition mats in USAG optional competition. I think you can have 16 inches total but I might have made that up in my head. It makes sense though.
If the gymnast is hitting the mat with her foot in any part the routine (such as a straddling skill), that will be a problem, so that would be a consideration when determining how and when to place additional matting.