I have a pattern and it says to test your tension before starting.
It is 10sts to 5cm (2") on size 6 needles doing either st. stch or pattern.
How many rows do I have to do? and is it suppost to measure 2" width.?

The wool band says 18 sts and 24 rows should make a 4" square.

Pesume I use the pattern tension.

Thanks for your help. Helen

Yes, that’s right. Go with the pattern tension. That will give you the size that you want.
You want 20sts/4 inches so cast on about 30sts and knit for a good 4 inches, then measure the sts over the center 4 inches. That measurement will be more accurate and will avoid the edge sts which are smaller than center sts.