Defining "tough non-conference games" as teams currently in the NET top 75 (Quads 1 & 2), I did a little research. Here's what my I came up with.
Of the 80 Power 6 basketball programs, 55 (68.8%) have played 3 or more non-conference opponents with a NET currently in the top 75. There are the 25 that have played 2 or less:
2 -- BYU, Cincinnati, Colorado, Louisville, Northwestern, Ohio State, Oklahoma State, Penn State, Providence, Seton Hall, Texas, TCU, Texas Tech, Vanderbilt, Wake Forest, Washington State
1 -- Georgetown, Maryland, Minnesota, Nebraska, Ole Miss, Oregon, Pitt, Stanford, UCF
The average number of tough non-conference games among the 80 Power 6 programs is 3.337, well above 2.
With regards to ". ... (including the top teams) have about 2 tough non-conference games", I did a sampling of some teams that most would consider top teams/programs current day and/or over the last 25 years, which includes every program that has won a national title since 2000. Here's that sampling:
7 -- Alabama, Purdue
6 -- Arizona, Michigan State, Wisconsin
5 -- Baylor, Florida, Marquette, North Carolina, Tennessee, UCLA
4 -- Creighton, Houston, Kansas, UConn
3 -- Duke, Kentucky, Syracuse, Villanova, Virginia
2 -- Louisville
Those 21 "top teams" this season are averaging 4.523 tough non-conference games, well over "about 2".