How do I count rounds in a spiral when nothing marks them?

In joined rounds the top of each round is a chain-1 and a slip stitch, so the boundary is a real, countable edge. In a spiral there is no such edge: the fabric is one continuous helix and the only visible clue is the spiral line itself, which drifts diagonally.

That drift is the key. Each round starts one stitch to the right of where the last one started, so the start points form their own column of stitches, each one sitting a little above and to the right of the previous.

Keep a marker in the first stitch of the current round and move it up every round. To count, follow the column of start stitches downward from the marker to the ring: each visible step is one round. On a 48-stitch round, that column is unmistakable because the steps are about 6 mm apart.

A faster method for tall pieces: measure. Ten rounds of amigurumi-tight sc in DK weight is 6 cm, so a 7.2 cm tube is 12 rounds. Check it once against a counted section and then trust the tape measure.

If you run out of markers, use a contrasting scrap of yarn laid across the round boundary and moved each time, or count the increase rounds only — a flat disc grows 6 stitches per round, so 36 stitches means you are on round 6 and nothing else.

What you should not do is rely on the pattern's stitch counts alone for a piece with straight rounds. Six identical rounds of 30 stitches contain no information about which one you are on.