The number that you are looking at pinned to someone’s name in a chat window, is their priority for that chat. You can find out which of you people have pinned to the top by hovering over “people” in a particular conversation, and then clicking on “view pinned” in the drop down. You can also see people’s priority by selecting their name from your participants list, and then choosing “pinned” in the drop down.
People who are pinned to the top will have their name displayed at the very beginning of your participants list. People who are not pinned, will have their name displayed with a number next to it that corresponds with the order in which they joined the chat.
These numbers are not persistent; they will change if somebody who joined later is pinned or unpinned. If you want to keep a particular order, make sure you pin your desired participants.