Yes, the limit is 20 activities per task. This is a client limitation. There is a list of the hardcaps for the Task system in the Wiki, they are:
Code:
#define MAXTASKS 10000
#define MAXTASKSETS 1000
// The Client has a hard cap of 19 active tasks
#define MAXACTIVETASKS 19
// The Max Chooser (Task Selector entries) is capped at 40 in the Titanium Client.
#define MAXCHOOSERENTRIES 40
// The Client has a hard cap of 20 activities per task.
#define MAXACTIVITIESPERTASK 20