How many questions can a user ask per day in a math.stackexchange?
Does the number depend on anything?
I tried to find the answer but unsuccessfully.
One user can only ask 6 questions per day. But there is also another limit of 50 questions per month (more precisely, per 30 days). So if you ask 6 questions every day for several days in a row, it might happen that for some period you will not be able to post questions at all.
These question quotas are not active on all sites in the SE-network. Here you can see when they were introduced on math.SE.
There are also some additional limits for new users and users who asked several questions which were not well-received. (Asking too many questions which receive many downvotes might even result in post-ban, which means that you are no longer able to ask questions here.)
You can find much more details in The Complete Rate-Limiting Guide.