Stack of item counts as one item on weighted pressure plates. For heavy pressure plates, 1 stack of 10 unique items only yields a signal strength of 1. Adding even 1 more item, as long as it is unique from the others then bumps the signal strength up to 2. Similarly if you put on 10 unique individual items on the pressure plate, you will get a signal strength of 1, but adding just another single unique item will then yield a signal strength of 2. 

This also applies to the light weighted pressure plate, but it triggers a signal strength of 2 with just 2 individual and unique items.

More testing shows that an entire stack counts as a single entity (also shown in screen shots)