It’s the number of unique machines (unique instances of snapd, and not e.g, shared IP addresses; nor multiple users on a single device) that have checked for an update for your snap in the past 7 days. By default, snapd will check 4 times a day, every 6 hours.
If a device goes offline for 7 days or more, it’ll be subtracted from the total. If it comes back online, it would add to the total again. If a snap is uninstalled, it would disappear from the total after a week.