John,

As Alan stated, the unit number for a militia regiment was tied to wher their militia district of their state was located.

For example, I live in Clarksville, Montgomery County, Tennessee and the county had 2 militia districts then (and now for that matter as all states still do) - the 91st and 92nd. Thus, the regiments of militia from this county were the 91st and 92nd.

Hope this helps.

Greg Biggs