So while my friends and I really enjoy the game, we have been failing regularly any type of game that involves interacting with npcs. So I have to ask, how many questions can you ask a npc per action. Are you limited to only ask one question per action that your character has? Any input would be greatly appreciated, we have a game coming up on Sunday and I would greatly like to conduct an interrogation without it taking multiple rounds.

When you see the action symbol on a button you can click on screen it means you will need to use one of your actions to perform it

If you want to search every search token and speak to every NPC and ask them all of the questions you can you will lose the game. The investigators are on a clock and will just not have the time to search everything and talk to everyone. You also might not have the time to defeat all monsters that spawn too.

You will begin to learn you are taking too long once the mythos phase stops pulling punches and will just start to hammer you with lots of sanity loss etc, sometimes you will not even be able to prevent it. Sometimes the mythos will be nice and tell you in a message that you running out of time and need to hurry up.

Exactly what @spoonman2084 wrote.

You have to find that sweet spot somewhere between investigating, fighting, and just simply getting a hustle going.