Pipelines.
Most commonly listed as "pixel pipelines" or "programmable shaders", the number of pipelines acts as the limiting factor on how many simultaneous shader programs can be run. Although cards do exist that do not have these shader pipelines, you are much better off getting a card with at least some pipelines. Unless you are severely price limited, you absolute minimum number of pipelines you should shoot for in a gaming card is 4. Those looking for midrange cards should generally shoot for cards with 8-12 pixel pipelines and the high end gamers should be looking to buy cards with more than 12 pipelines