We want to try the cloud product, but have following questions
Unfortunately there's no API for the Cloud version of Questions for Confluence right now. Once upon a time we did have one (and some documentation you can still find points to one) but a re-architect of the app on Cloud left it without a user-accessible API.
However, as far as I know there are no limits on the number of questions, answers, or comments you can have. We have many questions on our own instance at Atlassian that go very deep! One use-case example for us is generating questions to ask during on-stage events - each question for the person on-stage is an Answer, and people vote on which ones they want to get raised on-stage.
Daniel | Atlassian Support
P.S. I saw your question about Slack integration as well and am looking to get you an answer from a product manager!