Organizations which try to grow their communities as large as possible end up with less active members than those that set a clear target.
When you set a target, you treat members better. You have a finite supply. Your members are a precious resource. Your emphasis shifts from recruitment to retention. Your actions change from external promotion to internal recognition, co-operating and ...
How big your community should be?
First, estimate how much time it will take to keep every member active. Perhaps 5 minutes per member, per month. Divide the community manager's time by that number. If each member will take 5 minutes a month, then 2000 members might be a realistic target.
You will adjust these figures as the community develops. The community might become more self-sustaining. The community manager might not need to spend as long building personal relationships with members to keep them active. Members might keep each other active. You might add more community managers. You might decide to include a few additional members when you have more time to do so.
Your community should be as big as your community manager can sustain, no bigger. Set a target and focus on the members you have. Untapped potential is better than wasted potential.