How many 100 Watt solar panels can you run with a 25 Amp MPPT controller?

As many modules as will provide up to 25A of current at the MPP. I can't say how many 100W modules that might be because it depends on the module specs and the temperatures the modules will be exposed to. To ballpark it look at the module's Impp, correct it for the average high temperature and then divide that into 25A. That will give you the number of parallel strings you can have. Then calculate your string length to get the string voltage you want and you will have a rough idea of the number of modules. 

It will also depend on the voltage of the battery bank you are charging.