If I purchase a reseller plan from any host that uses WHM/CPanel, i.e. say with 2Gb of space and 20Gb of bandwidth, can I actually use the FULL 20Gb per month or is it only a percentage of the 20Gb that can be used for any given month? I hope this is clear. :)

With most providers, you can actually give out your clients more than you have allocated, as long as they don't use it all. And yes, that is per month :)

You can use it all yourself if you want. No rule that says when you buy a reseller plan you have to actually resell :)

If you do resell, you can still over sell your 20GB to your customers, and the total 20Gb is taken as a culmination of all your accounts.

Basically you get 20Gb, how you split it is up to you - if you even want to split it at all.