On the planes I usually fly (AA narrowbody MD80 & 757), in coach, there are two bins for every three rows of seats. The bins on the 3 seat side can stow 3 rollerboards (wheels first), and on the 2 side of the MD80, they can stow 2 rollerboards (sideways). That means that if only rollerboards are stowed, there's only enough space for 2/3 of the pax to stow bags (for 3 rows, 10/15 on the MD80, 12/18 on the 757). I suspect the numbers are similar for other airlines and aircraft.

So, if you board towards the end and the bins are full, it may not be due to bin hogs. That's especially true on routes where many have rollerboards (business-traveller-heavy routes), in the winter when many have heavy coats, and over the holidays when leisure travellers are toting lots of gifts.

I'm a believer in one item per person in the bins myself, until everyone has boarded. It's just common courtesy.