How many steel bars in one ton of 12 mm dia of 12 m length?

Since you're using metric measures, I corrected the question to read "tonne" instead of "tone." What needs to be determined is, are you talking about theoretical steel bars, or actual rebar sizes? In actual sizes, there aren't any 12mm bars; the closest thing is 12.7mm. If that's what you mean (that's size 4 rebar), it's weight is 11.952 kilo for 12m. So your answer, for one tonne, is 83.67 of those 12m bars at 12.7mm thickness each.