A young man writes in Action #297 (February 1963):
560 different Superman family comics may not sound like a lot these days, but back then this was very impressive. First, I estimate that there were about 960 comics featuring Superman/Superboy or his friends, so it's almost 60% of the total. Second, this was before there were comics shops, before there were the yellow-colored ads offering back issues for sale. I'm sure that Ward had to scrounge everywhere to find that many comics featuring the Man of Steel.
I checked ahead in the next few issues and didn't see anybody writing in to claim they had topped his total.