Please offer some advice.

Hello again.
The PDF states that there can be no more than 8 armies on the map at the same time. Does this mean that I can release a hero and 7 individual units and thus trap the opponent in their castles?

This means that you can deploy 8 armies (heroes with or without monsters, or individual monsters) onto the map, and you can also hide one army in each city or other object where this is allowed. And each of your opponents can do the same.