I'm not sure how much that really counts. According to [1], the time that Unix spent as a single-user operating system was intentionally very short-lived. Even the earliest version of the operating system that can be reconstructed today had multiple logins and processes, though only one could be active at a time. It seems that, by the time Unix was written in C, ported to multiple architectures, and spreading outside of Bell Labs, it was already multi-user.

[1] https://retrocomputing.stackexchange.com/a/10908