I have posted the Wargame Culture Basic Massed Combat System rules on the WGC Substack:
This Massed Combat system is designed to be used with any RPG system.
It is totally system-agnostic to facilitate Massed Combat encounters at the table with a minimum of preparation & in a short amount of time.
Your RPG system may have its own rules for Massed Combats, and these are likely to be more appropriate for very large, set-piece battles, or battles that take place ‘off-screen’.
Feel free to adapt these rules to fit your table’s play style and aesthetic.
These rules are formatted into booklet form, with a link to the PDF download at the end of the article: Basic Massed Combat System.