"I have many questions."
Translation:Eu tenho várias perguntas.
Muitíssimas means more than muitas. But it is a long word so many natives prefer to increase the meaning by giving emphasis with another word like saying "muitas ... mesmo" which means "really many, really that many". The correct way to say "muitas ... mesmo" is to position the object in between like "muitas maçãs mesmo!"
Kind of, questões gives the idea of writing questions, for oral ones we use perguntas. Everybody will understand if you say questões for oral questions, but it is awkward. It could also lead to the idea that they are hard questions, disencouraging the native from wanting to answer you. So i suggest you don't do that... Native here btw
When it comes to indefinite pronouns working as adjectives (such as “várias”) or numbers near a noun, I would always use them right before the noun, not right after it.
This is especially so in the case of "várias", because the word "várias" may be a indefinite pronoun working as an adjective (meaning "several") inflected to the feminine (no singular form is accepted in this case) or an adjective on its own (meaning "diverse" or "contradictory") inflected to the feminine and plural.
If you use "várias perguntas", the meaning is "several questions", if you use "perguntas várias", the meaning is "diverse questions" or "contradictory questions", not necessarily many. This happens, because in Portuguese sometimes changing the position of the adjective will alter its meaning.
Therefore, as a native Portuguese speaker from Brazil, “perguntas várias” does not sound natural, especially because we only tend to use “várias” meaning “several”, not “diverse” or “contradictory”.