18sts = 4inches

I am doing my gauge, yippie.
The pattern calls for 18 stitches to equal 4 inches. The yarn states 18 stitches will equal 4 inches on a size 7 needle.
I had to move up to a size 9 needle to get 18 stitches to equal out to 4 inches. (this is a garter stitch pattern, I counted the bottom bumps on the stitches)
I got gauge, should I worry about having had to move up two needle sizes? The sample looks good, doesn’t look loose.
So, in this case it is not a certain number of stiches per inch, but a total of 18 stitches equaling 4 inches.

18 sts over 4" is the same number of sts/inch regardless. You’re probably a tighter than average knitter. The needles suggested are where you start and you use whatever size you need to get the gauge. Some people knit loose and may have to go down 2 sizes. Also, instead of just casting on 18 sts, use 24 sts and measure the 4" over the middle. The edge sts are not always the same size as the center ones and tend to curl under so it’s hard to measure accurately.