Hey Sjoerd,
You can use the "limelm.pkey.setDetails" function to change the number of allowed activations.
In the C# web API project this would look like this:
int newActCount = 10;
//TODO: get the pkey ID using LimeLM.GetPKeyID()string pkeyid = "100";
LimeLM.SetDetails(pkeyid, newActCount, null, null, null);
You can download the web API from your api page (requires login).
Tell me if this helps.