Practically speaking, there is no set limit to the number of alternatives you can import into the tool. The backend of the tool can technically handle thousands of alternatives. However, keep in mind that with the increasing number of alternatives, the time required to import data, open models, and work through the decision process increases accordingly. Remember that people will have to rate each of the alternatives against the criteria that you have developed.
In addition, note that the combination of data elements that the tool captures (primarily: number of criteria, number of alternatives, number of participants, number of resource pools) is also a significant factor with respect to software performance and the size of the data set. You could have a large number of alternatives (several hundred), a medium number of participants (dozens), and a small/medium number of criteria (15) and the tool should be able handle it with no problem. However, if you started to deal with hundreds of alternatives, hundreds of participants and dozens of criteria, the performance of the tool would be affected. Also note that with large data sets, while importing and opening models may be successful, there are elements of the application that will be suboptimal. This can be seen in areas like sensitivity where dragging bars and redrawing may take a long time for each small change. Additionally, without an ability to search or paginate, elements like Alternative constraints become almost unusable to have to scroll through dropdowns of hundreds of items.