"I have many questions."
Translation:Jag har flera frågor.
For me, 'many' is 'många'. 'Flera' is 'several', which is less than 'många' - that is my feeling. (native speaker)
According to me; 'många = many' and 'flera = several'.
There is a subtle difference in numbers as someone pointed out earlier, 'several hings' < 'many things'
Both options are common depending on what you want to say...how many questions you have.