How do I make it so that its me vs. 10 enemy engineers on the blue team?
Solution 1:
Valve added a new, hidden cvar in the Meet Your Match update two years ago that makes bots always choose certain classes instead of just being random.
The cvar is tf_bot_spawn_use_preset_roster. You can apparently change this as long as sv_cheats is set to 1.
Edit: Having said that, have you tried tf_bot_force_class engineer ?