Shadowserver tracks botnet activity and the number of command and control servers. It uses a variety of metrics to slice and dice its figures based in part on the entropy of botnet infections. The clear trend within these figures is upwards, with a rise in botnet numbers of 100,000 to 400,000 (if 30 day entropy is factored into equations) or from 20,000 to 60,000 (for five day entropy).
Entropy of botnets is calculated on the basis that if no activity is seen from a specific IP for a number of days – either 30, 10 or five – then it is removed from the botnet count.
I’m surprised the botnet numbers are this low