My 870 does have and 18" barrel, so I would be looking to hold 6.
6 in the magazine, plus 1 in the chamber
Woud I require a different spring?
The Wilson extension comes with an extra-strength spring and a new follower. I have used these on several 870's and I think they are the best; most of the custom 870 gun builders use the Wilson extension. They are very sturdy and do not require a barrel/magazine clamp (but I use one anyway - but that's just me).
Just be sure to choose the right one, depending on how you wish to mount your sling: left side, right side, vertical, stud, or no sling mount.