Units that are not at full strength can be “replenished” for some money. How exactly does that work? Is it affected by the region the unit is in? Can I replenish as many units per turn as I want or is there some limit?
Replenishing units is not affected by region at all. The only restriction is if you have the cash to replenish that unit, and you may replenish as many as you want per turn.
The cost is affected by the number of troops that have been lost, where the total cost is the cost per troop for that unit multiplied by the number of lost troops. No matter where the troops are, the new recruits will show up two turns later. If some troops were lost in the meantime, the unit will still be replenished by the amount of troops that you paid for.