All you have to do is plug a Random Value node into the Polygon Index port, and then set the Mean Value and Variance. If you then play around with the ID (of the Random Value), you’ll get a different polygon extrusion.
Most of this tree is for setting the Mean and Variance. If I have an odd number of polygons, say 81, then a mean of 40 and a variance of 40 will cover the range of polygon indices (0-80). But if I have an even number of polygons, say 80, then a mean of 40 and variance of 40 might give me an index of 80, which is out of range.
To randomly extrude more than one polygon, just feed a bunch of IDs into the Random Value node, like this:
I increased the number of polygons to avoid getting the same random number multiple times.