RG, You're right on it. One instructor to four students is the basic rule. As posted on the Preserve's website. The ratio changes as the number of students increases. But, it is all laid out in their regs. Part V, paragraph 2 http://www.mohonkpreserve.org/index.php?climbing_instruction
What I find surprising is the size of an instruction group could be 20 students not counting the attending guides. The regs fortunately, won't allow that large a group to do anything more than TR or rappel.
One way or another, it won't be long before LC begins to see bigger crowds and/or the risk of additional regulation or restriction.