Usually you can edit it with the editor in Windows and choose "Any files" as target. But i heard that editing the .bat is not working with a result on every end.
So leave the .bat as it is now and use minplayers=xx?maxplayers=xx? command in the changemap URL or pls install the ServerEX. With ServerEx you can easily set a specific Number of Bots to your Server and you can change the count everytime you want. The change will be done when a new map is loaded and will stay until you change it. There is no need to use the "minplayers=xx?maxplayers=xx?" anymore after installing ServerEx. You can even add yourself as Admin with an Autosignin so that you don`t need to login per Ingameconsole manually in each round of a map. You can even see Teamkills and you can pause the game when you`ve been logged in automatically. It is also good for clan wars cause there is an enhanced Lobby feature and a feature for turning off the mounted MG`s which are not used in clanwars.
If you want to use ServerEX and need a bit help call me per Steam i can help you as good my english is.
Here is the Link to the Mutator:
http://forums.tripwireinteractive.com/showthread.php?t=84401