Is there a rule of thumb to tell when or if your narrative is getting too "crowded," when writing pulp?
Sure. Depends on what you mean by being "crowded."
Too many characters?
Too much going on at one time?
Too much develop for what should be a more simple story?
I don't think there's a rule of thumb as much as a what feels right to you as the person creating the tale. In general a pulp tale should be a fairly straightforward adventure story with a linear, easily progressed and followed plot. So, I guess you could say that when things start to get convoluted you're probably leaving the arena of pulp and moving into something different.