I usually put some in if they annoy me, but I'll take them out once they seem to start getting less messy. I think I got to a peak of maybe 4 at a time, but that was years back. I believe I only put 2-3 on my Ignore List for a significant period of time in the past year, though.
The "This user is on your ignore list, click here to see this post" message is more annoying to me than seeing someone around. I have to pay more attention to it than I would them. So 0, but if there was no message thing I would have a few.
This.
If the ignore list actually made people completely disappear I'd probably have a few on it but simply haven't got the self control not to click the message to see the post so I end up paying more attention to them than I would if they were off my list