Sponsor ads are displayed according to the display share you give to each ad. Your display shares must add up to 100%.
Google Cloud Platform gets 75% display share and Brella 25%, how often are the ads displayed for these?
If you set Google Cloud Platform at 75%, it means that all attendees will see that ad in the spot (the fourth tab in People's tab) for 75% of the time that they see that spot. So if they reload the page, they should see it 3 out of 4 times.
This is a direct conversion, so there is no formula. The ad in the above example is displayed 75% of the time for any given user in any given browser or device session. The other ad has only a 25% chance of being shown.
Whenever a user opens the app or refresh the page, the ad is refreshed based on your display counts. Because of this, the same ad can be shown 2 times in a row, even with low display share (however the probability of that is low).
Tip: The maximum number of sponsor ads that you can add within Brella is 100, so if you add 100 sponsors ads each would get a 1% probability to be displayed.