If you do not know, a Flawless move is when you do well enough on a move for the ring at your feet to fill enough to turn purple. You will need to do this 10,000 times in any mode. Since the longest song I have seen on the disc is just over 80 moves, you will need to play at least 125 songs. Run (Natural Disaster) from Dance Central 2 is about 93 moves and the longest song I have found from the imports, so you will need to play over 100 songs to get this anyway.
To make this go faster, turn off Freestyle from the options menu and you will gain around 10 moves per song. You cannot turn off Freestyle for songs imported from Dance Central 1, though. Dance Central 1 songs also have far fewer moves on average, so you are better off drawing more from the songs on disc and from Dance Central 2. This can be done on any difficulty.
There is no way of seeing how many moves you have gotten flawless or nice on, unfortunately. However, if you sit in the main menu it will give you random stats. One of those stats is "Songs played," which will allow you to get an idea of how close you are if you have a vague idea of how many flawless/nice moves you are getting per song.