This is an interesting question. It can be achieved by using some expressions to divide the length of the spline and find the number of 1s,10s,100s,1000s etc. Once this is know the value for each can be wired to a selector operator that picks the correct number. The numbers are then combined in the correct order using a compose operator to give you your final value.
I've also added a variable scale so that two arrow heads and numbers are not larger than the spline at the beginning of the animation.
Because it uses a few expressions it isn't the easiest of styles to understand, but I would recommend examining the attached file and taking a look at the Counting Numbers
tutorial which uses many of the same principles.
I hope that helps, please let me know if you have further questions.