each player requires about 5k/s of transfer in each way, download and upload, if you host 16 slot game server, full of REAL players.
lets assume you run listen server on the machine that is directly connected to internet, so your friend can join and play
if only you wo play, then server sends to intertnet traffic to your friend only, and you play over loopback (and it is not counted in to real internet traffic of your provider). it shoudl take up to around 2k/s in both ways for each one of you
now if you add let say 16 bots, then those bots will act as if they were playing also on localhost - so all traffic will be looped and will not be directer through internet connection.
but bots move, and the info about movement, shots etc needs to be sent to clients, that means another bots and real players. Bots will be getting traffic from locahost, like you. Your friend will have to get data too, so the network traffic for him will go up, something around 5k/s.
to make it more understandable
if you hsot 16 players server with 16 real players, you need 16x5k/s upload and download , that is 80k/s in both directions total (well a bit more but let assume its average).
Now if you host server for 2 real players + 14 bots you need to use only 2x5k/s, that is 10k/s in both directions.
If you play on the same machine you host game , or you are playing from lan machine, then the traffic is even lower, cause internet requires just traffic for your friend, that is 5k/s
so, you got 128kbit, that is 16k/s, you should be able to host about 3 persons from the internet, you on lan or the same computer you are running bots, and bots itself (their number rather depend on your hardware