What is the maximum number of players on a Minecraft/Bukkit server?

The number is set from the server.properties file and can be as large as you like as long as the server has enough bandwidth and memory to hold all the people.

If you're asking about a particular server, the maximum number of connections can be seen from the server list in game.

The only existing cap is the maximum integer size in java, which is 2^31-1 (2147483647).