The way I do this is to have a non-featurized “Any” slot type that contains an index number pointing at the current question. After asking the user each question, I use actions to increment this index number by 1. I also have another slot containing the last index (which is different depending on your number of questions you have each time). When the two are equal, I run an action telling the user that my questions are over.
These slots are stored in the tracker and can be retrieved easily using rasa-sdk. In other words, I have my assistant remember what question we’re on and not move to the next one until the user answers the current one.