Most patterns start with instructions to chain a certain number, and then the first row starts working into that long line of chains. Those first chains are referred to as the foundation chain.
When you start to crochet into the foundation chain, you generally skip a certain number of chains. Those skipped chains may or may not count as a stitch.
For single crochet, standard instructions would read: “Sc in 2nd ch from the hook and in each ch across”. That means you are skipping 1 chain (the 1st chain from the hook). This skipped chain is the equivalent of a turning chain and typically does not count as a stitch. So for a single crochet row:
A foundation chain of 6 chains results in 5 single crochet sts.
Number of Sc sts in Row 1 = Foundation Chain – 1
For half-double crochet and double crochet stitches, the turning chain typically does count as a stitch, so things are a bit different.
Half Double Crochet
Instructions would usually read “hdc in 3rd ch from the hook and in each ch across”. The 2 skipped chains count as a turning chain AND the first st.
11 foundation chains results in 10 sts.
Number of Hdc sts in Row 1 = Foundation Chain – 1
This is similar to half-double crochet. Instructions would usually read “dc in 4th ch from the hook and in each ch across”. The 3 skipped chains count as a turning chain AND the first st.
12 foundation chs results in 10 sts.
Number of Dc sts in Row 1 = Foundation Chain – 2
Notice that I say “usually” in these descriptions. While this is considered standard, there are times where it makes sense to do things in a non-standard way. Some designers, myself included, prefer to use a loose ch 1 for the turning chain on a half double crochet and not count it as a stitch. Other patterns may instruct you not to count the ch 3 on a double crochet row as a stitch. As always, I advise designers to only use the non-standard format if they have a very good reason for doing so.