That depends on many circumstances. Obviously, a manufacturer or a dealer will have many speakers that they are not using, and do not intend to use for themselves. Their purpose is for sales. Any number that they have room for and can afford to make or purchase and that they could reasonably expect to sell for profit is a sane number. Now, someone who is an end user, and only an end user (some are more than just this, as some buy and sell used gear for profit), would ideally have enough speakers to cover all of their uses, and not too many more (though a spare or two might be reasonable, if one has the extra space for them). I personally have 4 systems set up (one of which is a surround system), not including computer speakers (which, if counted, would make for 5 systems). I use them all, though not all of them are used very often. I have some extra speakers, though some of them were purchased for reselling, and some for projects that have not taken place; i.e., someone changed his or her mind about what was wanted. I have too many, and should get rid of some, though given all of the circumstances, it probably does not qualify as an insane number (though really this should be judged by others rather than me). [And if it is an insane number, many who post here are in much greater need of help than I am.]
An insane number is having so many that they cannot be used, and are just cluttering up one's space, making a person's life worse than it would be without them. Notice, this is space dependent, not a specific number. Another insane number of speakers would be a number that costs the person so much money that it makes their life worse than it would otherwise be. Notice, this is dependent upon individual finances, not a specific number.
Basically, if you are a hoarder (in this case, of speakers) who psychiatrists would generally regard as in need of help, then you have an insane number. Otherwise, you may claim to have a sane number of speakers. Needless to say, there is a gray area in this, where it is questionable whether one has a sane or insane number of speakers.
It is also worth mentioning that if a person's goal is the best possible sound, usually the proper method will be having fewer, better speakers, rather than a bunch of lesser ones. Many people would have had significantly better sound if they had saved all of the money that they spent on the extra speakers and just bought one really good set of speakers with that money instead. Many could still do this by selling off their stockpile of speakers and using all of the proceeds on one good set of speakers. (I write "set", as one might be looking for a surround system, rather than a stereo pair.) But with hoarders, the best sound isn't really the point.
