I've used 10# of blueberries in a 5 gallon batch with good results. They were frozen for months, thawed completely, squished in the Foodsaver bag and added to the secondary fermenter for a month or so.
I've also used black raspberries in a stout, never yet in a mead. 9# were added to the wort at flameout and steeped for 30 minutes. I'm going to have to make a second 5 gallons and blend the two, because 9# was waaaayy too much.
I'd start with 1# of raspberries in one gallon of mead. It's easy to add more later, but quite difficult to take it back once it's in there.