Is there an upper limit to the number of RVT?

I have five sub-levels in a persistent level, and three RVT Volumes are placed in each sub-level. Each time I open the persistent level, the RVT of two sub-levels will be invalid. If I delete any two sub-levels, there is no problem with the other three sub-levels. Is this due to the upper limit of RVT Volume?

Same question.I really want to know this too.