So I put my imperial stout (expected ABV 9%) in the fermenter on Wednesday (now Saturday). Pitched safale s-04 onto 10.5 litres of wort at around 22 degrees C. So far there doesn’t seem to be much activity. The lid felt pressured but no activity in airlock. It’s sitting at around 20 degrees C. Should I pitch another packet of yeast? Any advice welcome. Thanks.
Airlock activity is not a reliable way to gauge fermentation. Unless you have a perfect seal, you may not see activity during active fermentation. The only reliable measure of fermentation is a gravity reading. The only reliable way to tell if fermentation is complete (or has stalled) is to take gravity readings three days in a row and see if the gravity does not change. If it changes, fermentation is still in process.
Here's an article regarding airlock activity.