"I have many questions."

Translation: Jag har flera frågor.

[friswing]

For me, 'many' is 'många'. 'Flera' is 'several', which is less than 'många' - that is my feeling. (native speaker)

[lord_zulo]

Native as well. I agree, i would not translate 'flera' as many.

[ALXNDRH]

Jag håller med!

[Rycecube]

Which option would be the most commonly used one?

[lord_zulo]

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.