Depends on the upload Shadow. A 256k upload is good for around abouts 45 people comfortably, so take a guess from there. Remember the difference between upload and download
Also your CPU speed is not very important, anything 633mhz+ should do (lots of servers have been ran on less without lag problems). The most limiting hardware factor with a server is the RAM capacity. Just make sure you place your spawns carefully, and don't use more maps than you need to and you should be fine.
Apart from that its the geographical location of your ISP, as this will effect people's ping rates (just ask a few people to ping you, see what the results are). Id say anything under 90ms is good there.
/Leo
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