seek help for worker issue

[温柔的杀你]

I limited the worker to 2 for each civilization and city-state, but I found out that the workers of AIs were easily captured, and a lot of terrains were not improved. I change the worker to the combat unit, and can not be captured any more, but they may be killed during war. I have an idea, if the worked is killed during war, the AI can produce one more, but I don't know how to make it come true, can someone kindly tell me how to do this?