How many copies does MAG need to sell to be successful

[Reasonable]

I'd guess from what I read about development time/size they're probably breaking even right about now or just passing into small profits.

Unless the game collapses I think MAG will see enough LTD sales to be successful. I think if Sony gave it continued exposure, perhaps arranging some clan tournaments with some rewards, etc. they could easily help it stay relevant for the target demographic for the game.