I am a sister of a local that has been around for 100 years, so I can understand the importance of wanting to keep your sorority going... especially since it is a local!!!
I guess I am confused as to why you have a quota for numbers if you are local???...
Anyway, I understand your fear of having only 15 girls and loosing a lot during graduation... At one time my sorority had 3 girls, two of which were abroad/off campus (it was before my time but I have heard the stories). Anyway, suddenly we started growing and now I believe there are 25 or so girls (I'm an alumni now, so I dont have the exatct number
) with 11 girls pledging this fall... You can't always understand why or how you can go from being so small and exploding to 20-30 girls or vice versa, it's just the way it works sometimes.
My advice to you:
DO NOT JUST TAKE ANYONE!!!
If you do, you will not be upholding the standards and values your sorority has held for 73 years... Instead, have your current sisters talk with their friends classmates that you think would properly reperesent your sorority. Have these "potential ladies" come to an informal information meeting; no pressure, just a chance for girls to learn what the organization can offer them and what they could bring to your sorority. Also, don't just take freshman or sophmore pledges, look at juniors and even senoirs. Just because they are over half way through their college experience does not mean that they can't bring a lot of value to your group (I was a senior when I pledged... It was amazing!!)
Also, make a strong presence on campus! Host social events not just with fellow Greeks, but for the whole campus to attend. Or do something like a clothes drive or blood drive with your college. Or, do an event with another sorority or fraternity on campus, or just another group. Exposure it key!!!
I wish you
TONS OF LUCK 
and I hope I have been helpful... Let us know what happens!
With much Greek Love,
Meri