We play mostly T10000 games now, with a max of 8 people (it's a little crowded at our table then, playing with more wouldn't be all that great) - but usually just 4-6.
Starting chips:
T25 x 12
T100 x 17
T500 x 10
T1000 x 3
Blinds go up every 10 minutes, and the games usually last about 2-2.5 hours. Most of the time ending around level 11-12. Once we got up to 3 hours, with half of that being a heads up battle which I won
From memory...
1: 25/50
2: 50/100
3: 75/150
4: 100/200
5: 150/300
6: 200/400
7: 250/500 (I might cut this one out and colour up earlier)
--- colour up T25 chips ---
8: 300/600
9: 400/800
10: 500/1000
11: 700/1400
--- colour up T100 chips ---
12: 1000/2000
13: 1500/3000
14: 2000/4000
15: 2500/5000
--- colour up T500 chips ---
16: 3000/6000
17: 4000/8000
etc
I should also look at a cap on the blinds, based on the number of players. Wouldn't want things to get too high relative to the number of chips in play. Maybe also have a little longer blind levels near the beginning. Lots of tweaks to try.
I have a set of 400 chips, but don't really have enough for normal full colour ups. I could, but I was figuring on a range from T5000-T25000, and I wanted some T5000 chips (which occasionally get into play). I guess for this setup, you could go with T25x100, T100x160, T500x115, T1000x25.
But since you want to support 10-20 people, I doubt this would work for you. It's more of a special case for up to 8 people for me
