It surprises me that someone would take a chance and do a launch without first testing the conversion rates never mind not having all of the sales material ready.
I believe it's critical to have everything in place especially if you are bringing on new JV partners.
There should be a launch checklist and if not all of the steps are complete then the launch should not even be announced.
1.) Product itself is complete and has been tested.
2.) All pre-sale material has been created for the pre-launch.
3.) Sales letter or final landing page has been created.
4.) All affiliate and JV material has been created and uploaded to the affiliate resource section and tested.
5.) X number of prospects should have gone through the sales process to get a good idea of the conversion numbers both on the opt-in and on the final sales page.

What would you say a good number of prospects would be to get a good idea of conversion rate?

In my experience I think about 200-300 raw subscriber, i.e. adwords traffic etc. it doesn't really seem to change past that, what are your thoughts?

It's critical to test everything - even test email addresses to make sure they work as well.
There will be some unexpected bumps in the road but at least you make sure you did everything in your power testing what you could and being prepared.