MS Licensing fun: How many licenses does a single nonprofit "donation" actually give you?

I'm the sole IT person at a nonprofit with about 40 employees, and I'm looking at ordering the latest on-premises version of Office before they stop offering them as donations. As usual, the way licensing works is... less than crystal clear.

The thing I'm confused about is that it's volume licensing, but it looks like each donation only gives you one seat for one device. It's been awhile since we ordered any MS products (we're using Server 2012, for instance), but I notice that on the volume licensing site, each of our products seems to grant 50 seats.

What I'm trying to figure out is whether it was always like that (eg, each single donation = 50 seats on the volume license site), or if we just ordered 50 units of each product in the past (which I highly doubt). If I just order one unit, will I only get one license, or is it more like volume licensing by default you 50 seats by default (in case you need to trash old devices and reinstall on new ones)?