U could do some basic maths on it. The probability of having an autoimmune disease is 1 in 12. So if all else was equal the probability of having 2 is 1 in 144 and 3 is 1 in 2000. But the stats day if u have 1 autoimmune disease u are 3 times more likely to have another. So on that basis the probability of having 2 is 1 in 50 and of having 3 is about 1 in 150.
A word of warning. The estimates of 1 in 12 and 1 in 3 VARY WILDLY by different surveys and sources!
Anyway, I have two. Crohn's and Hashimoto's thyroiditis. The latter came about
a decade after my Crohn's diagnosis, but I wasn't officially diagnosed until 2013. My GI ran a thyroid function test in 2011 and found that my thyroid antibodies were sky-high.
Crohn's arguably isn't autoimmune I know. I suspect that applies to more small bowel Crohn's, though, and I had mainly Crohn's colitis. Or else I'm just talking out of my hat, which wouldn't be the first time.