If I want to make this for 40 people, is it ok to double ingredients?

To make in one go? Yes, most recipes work in a double batch. And don't work in multiples larger than 2.
Will a recipe that serves 8+ serve 40 when doubled? Doubtful.
Even If you get 10-12 slices per cake (a reasonable guess), you'll need 3-4x the original to serve49 people.