A good rule of thumb is to take the total number of members you believe will commit to participating... and add 5%.
If you know that 80 members of your church will commit... add 4 more books to your order.
If you know 300 will commit... add 15.
The reason is that you will ALWAYS have people who want to sign-up last minute, or have friends from outside the church who want to join in.
You never want to get folks excited about a campaign, only to end up not having enough books available.
As far as paying for the study guides, there are two ways you can do it.
One way is to have the church cover the cost for every member it thinks will participate.
This works out to be about $7-12 per member (or the cost of a couple cups of fancy coffee).
Members of the body tend to get excited when they see the church making an investment in them.
Many churches have made this same investment... and have seen it only make their congregation more generous in return.
If your church is NOT in a position to cover the costs for the members... that's perfectly fine. Many churches ask their members to cover the cost of the book.
Most of the time folks will not have an issue covering this cost.
One recommendation we make is that you always have a "giving bucket" nearby when you're selling the guides... that way folks can donate their change which will cover the cost of someone who may not be able to afford the book otherwise.
This usually works out very well... and allows everyone to participate.