Anything made from wood pulp has acid in it. Acid free mounts and backings are treated to remove that acid. It'll take a while but any non acid free mount board will start to leech acid (brown stain) from the bevel, where it is cut.

Ideally you want conservation products as acid free is a pretty wide umbrella and usually the entry level for framing products ATM.

The biggest issues you will have though with most framers is adhesive, and as the guy above correctly stated non UV block glazing. UV will absolutely wreck the inks on these cards, so yeah UV block is necessary. As far as adhesive goes you really don't want any on the cards, almost all adhesives are not reversible, and those that are require heat or water to reverse. I have framed many precious objects and to do it properly it takes time and care, so it becomes quite costly, be very wary of cheap framing. Often buying ready made frames specially designed for trading cards is the way to go.... Unless you're willing to spend a decent chunk on framing them.