This is a question that would basically answer itself after a thorough perusal of an introduction to car audio basics wensite such as
http://www.bcae1.com
The answer depends on two factors - the manner in which the amp was built and how low of a resistance it can safely accept in whatever mode the amp is to be used in and then the resistance that the speaker(s) is/are going to present to the amp when connected.
The answer to how many speakers can a single channel power adequately is however many (whether that be a single, lone driver or multiple drivers wired together) you can connect while still remaining within acceptable resistance limitations and adequate power output.
Ergo - you can take an amp that will output 100w per channel into a 4 Ohm load.
Let's say you have a 4 Ohm speaker capable of accepting 100w RMS.
That speaker by its lonesome is a great match for that one output channel.
You could take 4 speakers exactly like the above example and wire them to a final load of 4 Ohms and connect them to the same output channel. Wired properly each speaker, in this configuration, would see around 25w apiece.
Would this technically
work??
Yes.
Would any of those four speakers perform optimally?
Nope.
So there's no one single answer to your question. There are too many possible variables.