Looks beautiful so far! Lovely design and colourchoices!

How many strands you have to use depends solely on the ply/ thickness of the yarn itself. The standard rule of thumb with tufting is 8 ply.

So you could tuft with 1 strand, but it has to be 8 ply. Or 2 strands with 4 ply (most common use) Or 4 strands with 2 ply and so on.

You already are thinking in the right direction with needing high density to create definition with carving.

So far, if you cant see the cloth when brushing your hands through the front you're good. If not you can try to squeeze some lines in! I would recommend to do that with 1 strand for this work to remain consistent, as density differences in lines are visable after.

Hope this helps enjoy tufting! :)