The cap to for garrison resources is currently 1000. This has been the cause of some discussion over the cap, whether it’s too low or not. This of course stems from the urge to save resources up for when you would need to spend them for something useful/cool. Valnoth replied to a thread on the forums with the following:
Quote from: Blizzard
Just to be clear. The resource cap is a holding cap, not a weekly earning cap. There are no limits (currently) on how many resources per week you are able to earn, just that you can walk around with a maximum of 1000 on your character at any given time.We are working on more cool things to spend resources on.
So not only will you want to spend resources on cool things, you should be able to farm up resources (such as via the lumber mill).