Don't if there is a limit but, the more lookups you have, the less readability is good.
Probably a better practice to have multiple simple tMap (3 or 4 lookups).
I don't believe there is a hard limit. I've never "run out" of lookups that can be used, if that answers your question. In fact, I have built a job that was horrendously busy with lookups that I couldn't add another without it being impossible to see what was going where. Memory will likely become a bit of a problem with lots of lookups as well. How many were you thinking of adding?