It depends on your process. If you have AG equipment, use as much as you want.
I'm a stovetop guy with only one big pot. For awhile I was going on the "more is more" theory and trying to cram as much grain as possible into every recipe. The problem is that made my dunk sparging in a secondary 1.5 G pot much less effective (five pounds of grain doesn't leave much room for water there).
So for my system, if I use up to about 3 pounds of grain, I'll get about 60% efficiency. If I try to push that to 4-5 pounds, it drops pretty dramatically. So I end up using more grain and getting not that many more fermentables. And because I didn't figure this out right away, I had a couple batches I had to top up with some hastily-boiled EL DME just to get the gravity up to something appropriate. Which, depending on the mix of grains you're mashing, can have a pretty dramatic impact on the character of the final product.
I'm not suggesting there's a theoretical limit on how efficient your mash can be when you PM. There are plenty of other stovetop brewers on this board who seem to get great results even with far more grain. But it comes down to knowing your own process. Mine doesn't scale that great beyond 3.5 pounds, so when I'm building a recipe, that's what I shoot for.