(F) 17 How much weight do I need to gain between sizes?

I'm 5^9 and 110 pounds. Yes, Im fine if you're worried! Just genetically skinny. (Thanks dad!)

Anywho, I'm obviously trying to gain weight. So far doing well actually! I'm starting to lift. My goal size is 2 (near future), and eventually 4? But I'm not sure how much weight I need to gain to get to that. 15-20 pounds perhaps? More? Im currently a size 00 jean (long of course) sometimes 0. How many pounds have you needed to gain to change your jean size?

Thank you all!