I am browsing Steam Community Mods and I can't find a mod that does this.
Try the 'Legions' mod - a mod for stacked unit combat for Civilization V. It's very old, though.
This mod makes the following changes to Civilization V:
- Removes the 1UPT (one unit per tile) limit.
- Legions mod is completely modular and can be run concurrently with other mods.
- Attack I promotion- +15% when attacking.
- Attack II promotion- +15% when attacking.
- Attack III promotion- +15% when attacking.
- City Defense promotion- +25% when defending a city.
- Defense I promotion- +15% when defending.
- Defense II promotion- +15% when defending.
- Defense III promotion- +15% when defending.
- Shock promotions now give a standard +10% to combat instead of +20% on open terrain.
- Unit maintenance is reduced by 25%.
- Units in cities no longer get additional hit points, to adjust for the ability to stack defenders in cities.
This mod doesn't add any new UI to help manage the ability to stack units. So unit's still need to be moved individually, and there is no easy way to look at the statistics of stacked units.
The UI doesn't compensate for stacked units - So it probably isn't ideal if you a lot of units (20+). However, it's very good for moving smaller amounts together (Less than 10?).