I've always found it better to stock the clean-up crew on an "as needed" basis. In newer tanks, if 1 snail / gallon is introduced, more than 1/2 of them will likely starve to death before there's ever enough for them to eat. Same goes for hermits -- 1 hermit / gallon will just as likely end up in many of them starving to death. Even if 1 snail / 2 gallons and 1 hermit / 2 gallons are introduced, there's a greater chance of the hermits killing and eating many of the snails in a newer tank, simply because it's easier than searching for other food that's scarce to begin with.
To start, I'd suggest only a handful or two of each, even in a larger tank. While this doesn't allow for any money savings by purchasing a large quantity all at once or some cookie cutter package online, it shouldn't result in as many of the snails / hermits dieing. If you notice they aren't keeping up, select a species of snail or hermit that tends to target whatever the current nuissance algae problem is, then add a handful of them to the tank. Repeat as needed until the total clean-up crew is keeping up like you want them to.
Since adopting this kind of clean-up crew stocking strategy, I've noticed that all of our snails and hermits live longer. I rarely need to replenish their numbers. Sure, it costs me more per snail and hermit, but I don't have to buy as many of them and it's become quite clear that I don't need anywhere near 40 of each to keep our 90g the way I like it.
