I usually start the boil with 7g and end up with a bit over 5 after accounting for trub, equipment losses, and hop absorption. You probably won't need as much as 7g as each setup is different. I'd start with 6g and see how it goes. If you end up with less than you want you can top off without causing any damage. If you end up with more than you want, you could waste time and propane boiling it down to the required volume.
Having done your first brew, you will have a much better idea of what volume you need for the next brew.