This was a question we got from a customer. With iExpressions, the answer is surprisingly easy:

1. create a slider control on some null layer
2. set the slider to the start value you want to have
3. apply the Increment At Marker 1D iExpression to the slider. If you want to count up, leave the values at default, if you want to count down, set the "increment at marker" value of the expression to -1 before applying it.
4. link the text to the slider value with an expression (just using the pick-whip)
5. add markers to the null layer (not to the comp or any other layers). At each marker the number will change, depending on the value you've set at "increment at marker" (i.e. do a +1 or -1, for example)