I have roughly that many players at a time on my server. I went for a 16K map (A17.4). I strongly recommend using the Nitrogen tool (search the forums for a link to it) if you are going any bigger than 8K as the RWG generator tends to leave tons of empty biome bigger than that - It's not much fun exploring nothingness when there isn't anything on the outskirts to explore. The Nitrogen tool will get you more tweaks to make the map the way you want (full control over biome, town, city, river, and lake sizes and quantity among others. Be aware that we are averaging around 30 minutes for the initial client downloads before they can join the first time. Afterward, it's closer to normal join times (1-3 minutes depending on client). If I could go 20K...I would.