I don’t know the percentage, but a large amount of townhouses were converted to multi family rentals from 60s onwards, so that owners could collect rent. Often times configured as an owners unit and multiple rentals.

That’s also when many stoops were removed in favor of ground level entrance.

However, in the last 20-25 years, it’s been going the opposite direction, and multi families have been converted to single family.

Townhouses are really rare and coveted.