"How many M1 mac will still be functional in 20 years?"

Probably quite a few, MacBooks have had soldered SSD's for over 10 years now. My 2018 McBook Pro still has a perfectly functioning SSD. I still see people using 2015 and older MacBooks all the time. There is no widespread SSD failure issue after 10+ years of Apple soldering the SSD's.

For most people the SSD's are lasting longer than the useful life of the device.

Just because it's soldered doesn't mean it can't be replaced.

(But it's encrypted, so you'd better have backups because you can't read it off the chips.)