RE: How many layers of gum is too many?
Asking the question of the subject line is like asking "how high is up?" It all depends -- on the emulsion (ratio of pigment to gum to dichromate, the particular pigment and its characteristics and makeup, nature of the gum, dilution of the dichromate, etc.), plus of course the exposure, the paper, the size and the color of your socks.
And oh yes, the NEGATIVE ! (A flat negative might be trickier with multiple coats than a contrasty one.)
Meanwhile, my guess would be that if your emulsion is thin, well exposed, paper well sized, and lightly exposed -- AND you wait til it's nicely settled (hardened) before doing the next, you can coat away until you're bored out of your socks.
And here's an "also" hint -- if you're unhappy with any of it, even after it's hardened, give it a VERY long soak (I dunno, maybe 10 hours maybe 24, depending) and you can gently OR roughly take off the top layer, the top 5 layers, or whatever, with a brush, a spray, or whatever. Or if memory serves, a certain amount of household ammonia in the soak water, will also soften up the emulsion.
Which is to say, if anyone gives you a specific number on this, they are clairvoyant, and wasting their time in gum. (See if they have some advice on the stock market.)