Hi, Jacopo! If it is one submission that holds multiple variables, that will be on Add used. In this case, I would expect you to be closer to the 500 Adds per day scenario.
With the new team plans, we have rolled out a Usage page as well that you can start using to estimate the number of Updates your team consumes. Please note that the Updates shown will be slightly higher than what will be counted in the coming months as we do not plan on counting (a) editing in the Builder and (b) Updates used by unpublished projects
I will send a DM to look into this with you