No problem - I think it's safe to say you should be able to add as many as you'd like for a legitimate mapping need, with performance as the wildcard - that's why I mentioned the theoretical maximum vs. reality (performance impact/memory limitations on your machine, etc.). Giving an exact amount for the later wouldn't be possible, I do not believe, as it's very nebulous...
The data you load, what you are doing within the data frames, how much memory your machine has, etc. - there are many parameters that would come into play. But, like for map layers, the physical, theoretical limitation in the Esri code should be 2,147,483,647 - this is the maximum number an integer type supports. All other things aside, this should be your absolute max!