I would like to send new questions to specific people to ask for details we needed. Would it be possible ? With notification ? Would I need to have one form / project per people from the beginning on ?
What ODK tool and version are you using? And on what device and operating system version?
You cannot send a notification to a person's phone through the ODK Collect app asking them to complete a survey. You could email them a link to an Enketo web form to let them fill it out in their browser after seeing the email.
Will the questions be unique or different for each person? Then you will probably need a different form for
each.
ODK 2 could be easier to set up the data flows (showing them what information they've already provided). But will require work to customize all aspects of the system and set up the user database and whatnot. And will require the users to install and set up the required apps on their device.