Hi Morna
We have a link to a handy dandy bead counter here
LINK It is a great estimater with and without knots. However it claims only 3 6mm beads to the inch with knots. You did not post the size of the pearls, but this bead wizard can calculate with any size beads with and without knots.
I recently did two 6-7mm necklaces for my daughter to wear on the pearl walk, so they would nest. My first guess was 4 pearls and that made the second strand way too long. I now think 2 pearls off the longer one and back onto the shorter one, would work better, but haven't redone them yet