(You mean 10 to 1...?)
Well no, I was assuming, and said, that he was going to multiplex so that one column of one plane would be on at any one time. Now if you are going to drive it with 300 LEDs in one plane all on at the same time then yes it is only 10 to 1. However that means he will have to have 300 / 16 = 19 TLC5940 to drive it. While not impossible, there needs to be some signal buffering going on, proper construction and good layout. It is not something you are going to make on bread board.
It can drive 120mA per pin
Yes that is the current, now work out the power dissipation that will cause in the chip and you will find you can't run the chip at that current.