anybody got any suggestions about how to measure how many parties are using.
The replies so far seem to be little to none ,but sometimes a lot.
It's heavily dependent on the composition of the party.
The group I DM for (which only has two players) has an NPC Life Cleric travelling with them. She's the dedicated group healer, and she's built to be good at her job. Generally, if the group is using a potion, it's because she's fallen and they need to bring her back into the fray. Admittedly, that doesn't happen terribly often. The last time it did, the party was in a hallway in the fire temple of the Princes of the Apocalypse adventure, and they got blasted with 13 fireballs.
If she wasn't travelling with them, or if she was significantly less effective, they would definitely use more potions.
I'd also say it depends heavily on how active or reckless the party is.
If you run fewer encounters in a day, the healers will have more resources available per encounter. Therefore, they're likely to use fewer potions.
But, if the group is more reckless, if they have a habit of splitting up and getting hurt while being away from the cleric, etc. then you'll see them using more potions.