The requirements vary, depending on how many players you are wanting to host, but here are some approximate minimum server requirements for the number of players you mentioned.
16 players - 1.5Ghz CPU, 512MB memory, 320kbps upload speed with a MaxClientRate of 7000
20 players - 2.0Ghz CPU, 640MB memory, 400kbps upload speed with a MaxClientRate of 7000
24 players - 2.5Ghz CPU, 768MB memory, 480kbps upload speed with a MaxClientRate of 7000
Your upload speed and MaxClientRate will be determining factors as well. For most people, a netspeed of 7000 is fine, but if you want to set it higher than that, you will need more upload speed than what I posted above.
If you are planning on running SF maps, you may need more resources than what are listed above.