I will quickly show you how to get bots and still earn achievements:
- Make sure you have console enabled (you can check by going to settings for keyboard and clicking advanced).
- Start a Private match that is password protected.
- Open the console by pressing ` under the ESC key.
- Type ''tf_bot_add X'' replace X with the amount of bots including yourself and press enter.
- Type in the console ''sv_cheats 0''
- press ` again to close the console

And you are done, now there are bots and achievements count.
The achievements also include events like Halloween.
Sadly not all maps work for the bots, in some the bots stay in spawn and don't move.

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3498442654