15 piggies and 3 feeders, 20 piggies and 4 feeders.... so I'd say 5 to 1. If you have non-wood trees, then that kinda helps reduce the need for feeders... :)
I have 12 piggies and 5 feeders and at the time I wake up all feeders are full.
My friend though has 13 piggies and 6 feeders and since he doesn't play Glitch for month I collect his meat (which is always on max when i wake up) and feed pigs.
Just depends on how often you want to fill the feeders. If you play every day, you can get away with a couple. If you only log in once a week, then you'll need more.
Just another data point: 18 piggies, 2 feeders and 3 collectors. My collectors are full at least twice a real day (that's how often I check), however one feeder is usually full and one empty, so I only need one feeder for 18 piggies (to fill 6 collectors per real day).
One of my friends has 14 piggies and now has 5 feeders since I added them, it takes about 1 to 2 RL day to get them empty. On top of that, there were 4 edible trees out of 5 and they only ate off the trees when i'd harvest them. they're on about 3 times in a week or less, which is why I come over and fill them, of course, I haven't been doing it lately so the pigs may be dead.
I have 24 pigs, 10 collectors and 7 feeders :) The feeders take approx 2.5 real days to empty, while the collectors fill overnight ;) As someone else said, the biggest factor is how often you're on. If I wanted to be gone a week I'd need to invest in almost 14 new feeders :) Or some animal youth potion ;)
So.... I have 22-27 piggies generally (depending on how long I am gone ;) and I have 13 or 14 feeders, I don't recall atm. I'm thinking of adding a few more to make sure they stay full because they are usually empty, and piggies dead, if I am gone longer than 4 days.
I have 10 collectors as well, and they fill overnight.