64 teams in the NCAA tournament in 2000
68 this year. But normally its 64.
A total of 8 teams played in the 1939 NCAA basketball tournament. Oregon won the NCAA national championship that year.
64
In 2013, assuming that's what you mean by "this year," the NCAA Men's basketball tournament included teams from 31 states and the District of Columbia.
The NCAA tournament expanded from 48 teams to 64 in 1985 and increased to the current 65 teams in 2001. There are some who propose increasing the number to 96, but the NCAA tournament committee is not considering that option at this time.
North Carolina
1. University of Illinois - Urbana Champaign
4 2012
1985
1985
This year, the 2012 NCAA Tournament has four teams from Ohio in the Sweet 16: The Ohio State University (Go Bucks!), Ohio, Cincinnati, and Xavier.