How to go about cooking changing this Lamb Raan recipe for more people?

https://www.dishoom.com/journal/recipes/dishooms-lamb-raan-recipe/ I am planning on making this recipe for a group of 20 people. The recipe is for 4 people. There'll be another main dish to choose from, so I think making 3-4 times the recipe. I have a standard oven (https://www.hotpoint.co.uk/hotpoint-class-2-sa2-540-h-ix-builtin-oven-stainless-steel-f100149/p). I can get 2 legs of lamb or 3-4 half legs- but I'm not sure how this will this change the cooking instructions and time. Any advice on how to go about it? Thank you, I am a novice to cooking lamb! submitted by /u/hebejebee