How do I scale this recipe to serve 4 to 6 people????

I typically go with the lower number when a recipe says it serves a range (1 to 2, in this case). So, if you want to serve 4 people, I'd quadruple it. If you want to serve 6 people, I'd multiple all ingredients by 6.

Depends on how hot your chilis are. If they are mild, then you can go for the straight 3X. But if they are hotter, cut back.

I would simply triple or quadruple the recipe to serve 6-8. Use a larger pot.