Thank you notes for Co-op

I am currently co-oping with a pretty large automotive manufacturing company. My department is large and as an Engineering Co-op, I have worked with many different people across the department, in multiple different “sub-departments”. I want to write thank you notes to a couple different people that I have worked with this semester, because I greatly appreciate how much they have helped me. I am planning to write one to my mentor, my assestent manager (my supervisor), and my manager for sure. I also want to thank a couple of people that have helped me and without their help I would not have been able to complete my final project, nor have been so successful in that project. There are a couple others that I feel I should write thank you toos, but I am not sure if I should or not. I will be returning for another co-op rotation in the same department during the summer, so I am not sure if that changes things.

I read somewhere that I should write thank you notes to everyone who knows my name. I have worked or interacted with many different people, whom all know my name, but all of those I do not believe need thank you notes. I may write to more people after next summer (that should be my last semester of co-oping).

How should I decide who I should write thank you notes too this semester? Should I write one to each of the engineers in my group (4, 1 is my mentor who is getting one)? Of just those that I feel have helped me greatly this semester, and wait to write one to each of them after my next semester? Anyone else I should include in my list of who to write to?

Thank you!