In fact it is 10 activations, meaning there is the possibility you could have a situation where you accidentally use up two activations on one domain. Then you are in the hands of customer support to see if they will give you that activation back!
Most other services I use for Wordpress simply track the number of sites you have the plugin avtivated on at any one time. This allows for a more flexible way to manage your domains.
Activations scare me, as I have had situations with other products where I have used up extra activations accidentally! Is there any chance you will change this system going forward?