There are a total of 7 Chilly Gnomes that you need to find for the "Search Chilly Gnomes" challenge in season 7 week 6. There are more than 7 Chilly Gnomes around the map, but you only need 7 of them to complete the challenge. This video shows the easiest ones to get in the most direct route.
When you successfully find and collect a Chilly Gnome, it will come up on the screen saying you have done so. This video includes one of the fastest routes to get all 7 Chilly Gnomes, so you could even do it in one game.
Do note, you need to win/lose the game for the Chilly Gnomes you've collected to count towards the challenge. You cannot just quit.