What exactly is basicity?
The issue is that basicity has a couple of meanings and you have to be able to tell from the context what it means.

The first is logical, and is used to describe how basic a base is. That is the trend that you mentioned about the Group 15 hydrides. (In my experience, this is by far the more common usage of the word.)

The second is somewhat confusing, and it is used to describe how many protons of an acid can be removed by neutralisation with base. For example, $\ce{HCl}$ has one acidic proton, so it is monobasic; $\ce{H2SO4}$ has two acidic protons, so it is dibasic.