I'm wondering if anyone knows if there is a limit to the number of different activities that can be scored? (i.e. mail open, page viewed etc...)
This is not RTP, it assumes that Marketo already has the e-mail address.
I do not know of any hard limits. I think the limit is given by the number of smart campaigns Marketo can process, which can be easily several hundreds.
I suggest you move your post to the Products section, where you will get more responses.
Pavel is right twice
There is no limit, excepted Marketo's processing capability if you are running a very large number of trigger campaigns that fire simultaneously. I know of leads that have thousands of activity lines, and all of them have been score. My own lead in my own Marketo instance has 98000 activities...
And also, read this: Getting the most of the community when you have a Marketo question
I have seen scores in the tens of thousands vs our usual 750 threshold.
It all depends on how you have set up your scoring programs.
No limit for sure.....