spawn only one when a ravager is close or raid happens

I am trying to change what spawns in a raid. So I can add the Red Villager to every raid that happens on a village. I see that a ravager will only spawn in a village raid, & we can detect if a ravager is close by, but that would run every time you get close to a ravager & I only want one per raid to spawn.  How can I code this?   Make my custom mob known as the Red Villager spawn once when a raid happens or once for each ravager that exists in a raid party.  I don't want to end up with too many spawning in & making the raid too much harder but I do want to add some custom mobs to the raid party.