I think most REs play fast and loose with IUI numbers as they are (presumably) different depending on condition and there is (almost?) no peer review by condition. I spent a lot of time reviewing IUI papers and crunching numbers far and wide and it is so frustrating!

For donor sperm cycles in absence of any other issues in the woman up to 9 IUIs have been found to have similar pregnancy rates (somewhere between 15 and 20%) but cumulative does not mean 20+20+20etc but rather if you start with 100 people 20 will get pregnant first cycle, of the remaining 80 20% will get pregnant so that is 16, of the remaining 64 20% is 13 etc... so REs who say 3 cycles a 15% is 35% are mistaken.