2 extra sirens?

I have 3!

You can have up to 100 various components with your SS3 system - only one base station per account, however (and 10 cameras/recording subscription with the interactive monitoring plan).

Actually if you look at the information about cameras, it says unlimited amount.  In the past, SS reps have said up to 10.

https://simplisafe.com/choose-monitoring2

So up to 10 definitely.  

I have found that 4 cameras is more than ample for my 1600 sq ft. house, plus the video doorbell.

I do have 4 exterior cameras from a different company.

^ but,  "doesn't matter how many cameras" and "unlimited recording" are two different things.  I don't remember a post, if there was one, about more than 10 cameras being ok.  If I missed this somewhere, my apologies.