How many speakers can you install in M1

You need to not go below 4 ohm load on the speaker outputs. Per the note in the install manual:

OUT 1 †† for speaker(s) ONLY (4 to 8 Ohms).
If connecting more than 2, use series/parallel
wiring to avoid going below a 4 ohm total load

So, Two 8 ohm speakers in parallel are supported. For more you have to wire in series/parallel combination. to stay at or above 4 ohms.