[SpaceMonkey]

For 20 people, starting with 8/8/4/2/1, you'd only need:

160 Green T25
200 Black T100
120 Purple T500
100 Orange T1000
20 Grey(?) T5000

Or 40 T5000 chips, if you really felt the need to color up the Orange (not necessary IMO)

If you want to start with 12 Greens and 7 Blacks, change the first two numbers to 240 and 200.

And if you want to stick to the 12/12/7/5/0 you could get away with:
240
240
200
200
0-40

But that still doesn't prevent the need from coloring up. It actually increases it, because having all those greens in play will slow things down eventually.

As for blind lengths, unless you have dealers, I'd recommend 40-45 minutes for this event.