OnActivate is triggered when you change focus of Forms of you application. If your app. has only one Form then it is triggered only one (after OnCreate).
I'm not in front of code at the moment, so can't confirm for Lazarus, but I'm sure this isn't the case. Sure, if your application retains focus all the time, then OnActivate will only fire once. However, if you switch focus between different running applications, then when you switched back to your application, the OnActivate would fire again for that form.
BTW, if anybody wants to update the wiki - that would certainly help others looking for the same info...
I'll run some tests on this tonight and do that. There's another minor change to the wiki I want to do tonight, so while I'm there anyway...