There are many factors that may come into play - the number of members and content items does matter, but not as much as current load, which is a combination of how many users are online simultaneously and what they're doing. Say, if your site is centred around feeds, the load would be higher than if it was focused on Discussions only. Or, say, community doing multiple video uploads with transcoding is very different from a site that is mostly focused on blogs/embeds/comments.
So, if we average those factors, I'd say that yes, you may conceivably have about 1000 online users with a good dedicated or cloud server, without going into, say, separating database and web server into separate boxes or clusters. Once you grow beyond that, separation would help you scale further.
1000 online members may happen on the back of as little as 20,000 registered members when the site is young and popular/viral.