I'm certainly no expert, but that won't keep me from putting in my 2 cents
AS mentioned before - good idea to call your NPC area adviser - she can help you with all this.
In my opinion, I think total should be a number that the chapters can reach reasonably plus a little room to grow. (considering where your at i'd say around 30 - that's just a huge guess tho)
You don't want it to be too low to prevent from competing with the local chapters.
Definitely play up the national advantage!
National is great because:
we have regional/division conferences
we have an extended alumnae support system
we have chapters all across the US
we have scholarship opportunities
etc etc (but never never ever bash another sorority!)