Mega if you're using PPS and Single if you doing PPD.
PPS seems to work well with Mega threads if you're able to build a fan base. If they keep downloading from your thread, they know you got what they want and chances are they will pay for a subscription if they don't have one already with your file host.
However, if you do PPS, I wouldn't use more then 2 mirrors, but if you're content isn't something that is easily removed due to DMCA laws, then I wouldn't use any mirrors at all.
PPD is better with single threads because it gets more exposure with SE (well, depends on a few other factors too) If you going to earn from downloads, it actually better to have many mirror hosts instead of a single one.