I actually just found an organisation online that wanted volunteers, and then emailed them to ask if I could join them for a few months. Whether you want to volunteer is something that depends on you. If it's something you're interested in, fits your career plan, etc, then go ahead and do it. There's no need/point to volunteer just because you think it looks good- adcom members can smell such attempts from a mile away. Besides, many of my classmates have not done volunteer work before- it is certainly not a prerequisite.
That being said, if you're interested, there are certainly many organisations out there, that you can easily pull up with a google search. I would suggest thinking about what kind of volunteer work you want to do (education? environment? etc) and where you want to be, then looking for organisations that fit your needs.
Sorry, I do not have the numbers on how many waitlisted candidates were admitted. In a sense, these numbers are also not very relevant, since whether you get admitted depends on who applies, etc.. And the waitlist size tends to vary a lot from year to year- at least, a lot more than the application numbers.
Some key learnings: 1) highlight your LEADERSHIP and IMPACT in your community. How have you made the world a better place? 2) what exactly will you do with this scholarship to benefit all your fellow classmates, as well as others you might be targeting? Again the focus is on community involvement. 3) "I need the money" is not a good topic for an essay- that's what loans are for.
The second category. Most people tend to be decide that they're ready for business school around the 4-6 year mark in their career. So the bulk of students being within this category reflects the distribution of the applicant pool, and not the school's own preferences.