"This is one study from the project" would make more sense in English.
אחד מתוך one out of (a group or collection)
Why not "only one research" ?
Because that's not natural English. "Research" is a collective noun. It's non-countable, which means you can't put a number in front of it and you can't make it plural.
What exactly does מתוך mean here?
"Out of", exactly as the translation says.
It's very odd English: it sounds as though the study was one of several removed from the project. I've reported it as unnatural (to say the least.)
I put "This is only one study within the project." which sounds better and was accepted.
Agree no one speaks like that