Megumin's cuteness is a miracle of the universe, it behooves you to create as many little megumins as humanly possible.

With back-to-back pregnancies every 10.5 months and assuming she hits menopause around age 51, you could get around 43 births out of her.

Statistically some of those will be twins, with the rate generally increasing with age and height/weight. Using data from https://www.cdc.gov/nchs/products/databriefs/db80.htm you'd get an average twinning rate of 44.1 per 1000 births. It would be on the low side initially, but as her body fat percentage rose from pregnancy it would even out. With those numbers you'd expect to get 2 sets of twins, but if she has a family history of twins that could rise to around 4-5.

If you gave her certain fertility drugs (specifically gonadotropins), that would increase the chances of multiple birth substantially to 3 in 10, with around 5% of those being triplets or more. In that case, you'd expect 13 sets of twins, with one of them potentially being triplets instead.

So you should have 45 children with Megumin if you do it naturally, and 57 children if you use fertility drugs.