I recommend starting out small and seeing what kind of workload your UXers can reasonably take. A lot depends on how your company operates, the scope of their projects, how much expert industry knowledge they will need to learn, etc. Be sure to define the scope of their duties well. As mentioned in previous (excellent) posts, there is a LOT of misunderstanding about what UX is. You will need to keep them from being treated as re-labeled graphic designers.
But honestly, more important than how many UX people you hire at first is whether or not you have made space in your company for the UX process and have adequate buy in from the right people. If, say, you bring in your UXer(s) half way through a project rather than before development actually begins, it won't matter if you have an adequate team or not. UX, and especially UX research, needs to be done before all that. And preferably the user needs uncovered by researchers will help to influence the scope of the project just as business needs would. But that often takes considerable buy in from the company that isn't always there.