There is no specific limitation.
​
However, it’s not recommended to run more than one campaign test simultaneously.

Why? 

Because the more tests you run simultaneously, the more difficult it is to understand which variation of which test ultimately impacted positively or negatively the journey of a customer. As a rule of thumb, we always recommend our customers to run Experience tests instead of multiple Campaign tests simultaneously whenever they want to create tests to ultimately evaluate funnel & holistic shopping experiences.