There is no best number. For me, I get about a 5% conversion rate for search engine traffic and about a 3% conversion rate overall. But there is a difference between members and actie members. I would rather have 1 really active, dedicated member than 100 members who registered and never visited again.

Don't judge success on numbers. Is your community growing as much as you'd like it to? Has activity slowed down recently? Browse this forum for other member-boosting and community-building activities for your community.