Regardless of the hard limits imposed upon you by the engine/language you choose, the number of players online will be determined by your bandwidth, your network infrastructure on your sever backend, the structure and size of your data, how close you will need to ghost that data out to all clients, how you have implemented things such as physics or particle effects in your netcode (client-side only with terminal states replicated out to clients accordingly being the best so as to not crud up the pipe with replicating the exact pattern of falling boxes but rather the necessary landing area of the boxes), your server farm, the zoning algorithms across your master servers on the farm, etc.
There are a huge number of considerations, most of which can take the 256 limit down to 2 with 1 frame-per-minute depending on
how you are creating your MMO.
I'm half as useful as a box of vomit.
