I use linen and one layer of tear-away stabiliser - for this biscornu as well - and it works fine for me..

good question

Anything over 10,000 stitches, Floriani recommends 2 layers of tearaway or I layer of cutaway. (Don't hold me to this - I'm working from my memory of their instructions.)

It isn't so much how many stitches there are for cross stitch. If the stitches are entirely covering the design area you will need different stabilising than if the design is in a more traditional style. I have stitches some of my own designs in a traditional style without stabiliser at all. Instead I have used linen and starched it before stitching. The one in the pictures, I used matching bobbin thread so it looks good from both sides.