I’ve counted five: the one he had with Margaret; the one he had during his year as a widower; the one he moved into when he married Kathy; the one they moved into when Terry returned from Europe; and the one from the last couple of seasons.

I wonder why they moved so often?

I've wondered why they felt the need to redo that apt. set so many times. Seasons 1-3 was the first one, with some updates from season to season, season 4 with the full stairway, seasons 5-6 with the 4 or 5 step stair, season 7 a new one, season 8 a new one, seasons 9-11 the final one, with an added stairway in season 11.

So there were 6.

Danny was "show folk" and probably had to move out in the middle of the night from one place to another to skip paying the rent. He must really not have been the big shot he convinced everyone else he was. Po Danny Boy.;)

Maybe this is why we saw him dragging Kathy and the kids out at 3 am.

When we saw them again in 1970 (“Make Room For Granddaddy”), they had moved to yet another apartment.