Shower Door Installation

I am putting in a new shower. It will be a kit with 3 walls and the pan. For the shower door, do I need to put two studs (one for each side)behind where it will go? If not, then will the fiberglass in the walls be secure enough to hold it?

There was no stud with the old shower as the door was secured with an achor that went through the tile and sheetrock.

Thanks

Plastic anchors frankly are fine for this, because there is very little stress. But if doing from scratch, why not block it up. I would for sure put in blocking ( 2X6 on their side) in several places approriate for grab bars, possibly horizontal and vertically oriented bars. Figure out where someone could possibly need a bar, and block it in. This will save you tremendous headaches later on.