We had a number of very small NM classes. When I was an active, we participated in FR in the fall, sometimes did a COB rush a couple of weeks into the fall semester, and did COB in the spring. FR yielded mixed results - one year we had 11 NMs, and the next we had one (hey, not my fault - I was a rho chi that year
). COB usually yielded one or two NMs. Small NM classes were the norm for us, but of the four other NPC chapters on campus at the time, usually 3 made quota (which was usually 30ish) and one missed quota by a small number and had a small word-of-mouth-only COB rush in the spring. (The chapter that missed quota wasn't the same chapter every year.)
If you're looking at possibly having a NM class of one because you only have one spot to fill, probably the best thing to do is to identify a very small number of women who are already friends with people in the chapter, invite them over to hang out at the house or invite them for coffee or lunch or whatever, and then choose from among them (or choose not to extend a bid, if none of them is the right fit). No need to advertise and hold a bunch of parties to fill just one spot.
Then, the trick is to make that one NM feel just as special, just as wanted, as if she'd been part of the NM class you took in formal recruitment. We always had a full I-week, and a full initiation that the entire chapter was required to attend (as opposed to "Suzy's getting initiated on the 12th, whoever's available should come to room 2-203 at 8pm" - missing ritual was serious stuff whether the NM class was one person or twenty) - and at initiation the NM's big sister usually went through alongside as if she herself were a new initiate.
There are several arguments for bringing in that one NM. Rushing, educating, and initiating just one woman may seem like a hassle. But if you think about it, you are giving a woman a chance to experience greek life, and specifically, to be a part of your sisterhood. And from a business perspective, you're getting dues money out of her
.