If you just need to know the number of nodes in any particular month, you can use Super Search to find the node id of the last node until any date. Most node ids are allocated to actual writeups, whether replies or root nodes: reaped spam nodes are included, and a few new pmdev-only nodes (posts or pmdevnotes). There's no such easy way to find out the number of root nodes (or the number of sopws, that is almost the same number) in a time interval.