One or two day shoot is good question. A lot of the folks I know will only come one day, especially if weather is not good or their buddies are not going. Travel, cost of gas, family time, spring break, etc. can all have affect.
I know the Cedar River club out by Black Diamond put on first ever two-day trad event last October which had fair attendance on Saturday. On Sunday, I was one of only 3 who showed up.