As far as I have been able to tell, Numbers is extremely versatile and dependable. I've built dozens of clocks with it and have relied on it for literally hundreds of other templates (even though it's always behind the scenes, so to speak.)
Numbers, at first drop, is a frame counter. Whatever length your *Motion* project is, whatever the frame rate, Numbers simply advances one value for every frame that passes. You can check this out by creating a 10 second 30 fps (300 frames total) project and a 10 second 60 fps (600 frames total) project, then drop Numbers in each and check the default animation — it will be 0-299 in the first and 0-599 in the second. Altering the End parameter (or publishing it) will mean the Numbers will count from 0 to your chosen end value until the last frame the Numbers object exists. If you want to stop the counting halfway through your template, then, when Animate is set to On in Numbers, you will have to put an Out point on the Numbers generator at halfway through your project. It should count from 0 to your selection, but as soon as the out point is reached, the Number on screen will disappear because it no longer exists.
In all the templates I've built that use Numbers, I have always turned off Animate. Once you turn off Animate, Start and End will change to Value. Instead of trying to keyframe the Value (for your purpose), it is *much better* to right click on Value and select Add Parameter Behavior > Ramp. You can set the End Offset to the number of frames (in your Motion Project) divided by 2 (or the halfway point) and publish the Ramp > End Value. Leave Curvature at 0%. If you also publish the Ramp > End Offset, then you can actually set different time points at which Numbers will stop counting. Since End Offset is increased from the last frame, it is usually better to add this parameter to a Rig Slider. In the Slider, set the 0 Slider value to End Offset 299* and the Slider 100 value to 0. [*presuming your Motion project is 30fps and 10 seconds long, otherwise the Slider 0 value will be the total number of frames in your project MINUS 1 because zero counts as one value!] If you publish the Rig Slider, then the 0 - 100 values will represent "% of length of clip" and you can set the end of counting by a percentage position in the clip.
And it only gets more interesting from here!
