Which MIDI keyboard are you using? And, are you using ST3 or MP2 as standalone programs or within a DAW?
Some MIDI keyboards have a split function, so that, for example, keys up to a certain note will send notes on one MIDI channel and keys above that note will send notes on a different MIDI channel.
If that feature is used then you could play 2 different instruments at a time; otherwise you can only play one.
If you want to play several instruments at once, then you need a DAW. Did you look at any that I listed above? Here is an example, using 5 separate instances of ST3 to play 30 different instruments:
Big pic:
https://i.imgur.com/FWIue9h.pngThe DAW sends MIDI on up to 16 different MID channels to each ST3. The yellow and green blocks are previously-recorded / edited MIDI notes.
If I want to add another instrument I add a track to Record my MIDI keyboard-playing, connect it to an ST3, load the new instrument into that ST3 and click Record. The DAW plays back the 30 existing instruments, records my keyboard playing on the new track (and plays that instrument too).