The key to scaling and sustained growth isn't to have a large number of volunteers with similar job descriptions.
The key to scaling is to to have an increasingly large number of volunteers each responsible for their own areas of the community.
If you want highly engaged volunteers, you need to give them ownership over elements of the community. You don't want a large number with similar, overlapping, roles. You want voluteers who need the responsibility, power, and recognition that comes with this ownership.
Over 10 years ago, I spent 50hrs+ a week volunteering on a community. I had to, I was responsible for a large chunk of it.
From an early stage you need to identify the people that write knowledgeable posts and/or have passion for a particular topic. Invite these people to take responsibility for that topic within the community.
You might create categories/areas of the site for this. You might simply give them responsibility for initiating discussions, responding to discussions, and soliciting contributions from others on this topic.
Take the FerrariChat community below.
Here you want to have a volunteer responsible for each specific model discussion. They stimulate regular discussions, encourage people to respond, create content about what's happening in their area of the community, and recruit people to join.
There are no shortage of volunteer responsibilities. It doesn't solely have to be category-based. We covered a few last week. You can ask members to write an opinion post every thursday. Or a summary of the news beat for their niche every Saturday.
A volunteer can host a weekly live discussion on a specific theme related to their niche.
There is no shortage of areas in your community volunteers can be responsible for, but they do need to feel responsible for it.
You can begin right now. Find one person within the community that has a passion for a particular niche, and ask if they would like to take responsibility for it.