The amount of questions you can ask is technically limited only by the amount of Resource Credits you have. But please remember, even though Musing does not show a flag option, spamming can lead to you your posts being tracked down and flagged through other interfaces -- these posts still abide by STEEM-wide rules.
General STEEM etiquette suggests up to 4 blog posts a day. It seems reasonable to extend this suggestion to Musing as well.
If asking questions is out of your curiosity to know better and more then it is okay, you are welcome to ask questions.
But the dark side is that in an incentivized platform, this leads to spamming too and I am sure that some people are just aksing questions to get upvotes and the system will take care of that, otherwise it will be flooded with spam questions and answers.