Yes, a Game Overrides A/B test supports up to 5 variant groups in total, this includes the control group, so you can add up to 4 treatment variants alongside it. Once you reach 5, the Add a variant group button in the Unity Dashboard becomes disabled.
This cap is in place to keep each test statistically meaningful: splitting player traffic across more groups shrinks the sample size in each one, which means a much larger total player base and a longer runtime to reach a reliable result. If you need to test more than 4 variations of something, consider running them as separate, non-overlapping tests instead, see Best practices for more on avoiding overlapping tests.
Please reach out to the Support Team if you have any further questions about this.