Just a second already. These folks gardened the land. A family used about one acre. Then we have 25,000 sites identified. If it were only a family per site this would indicate a 250,000. A more likely figure would be 2,500,000 with careful field work strongly indicated to establish real population level for the region.
We know already that large populations in the Mississippi valley was completely plausible with the agricultural tool kit. This locale proves the reality of this conjecture. I have suggested that a number between twenty five to fifty million was at least possible for this whole area of the Mississippi valley.
The corn based agriculture allowed dominating densities and food security as well. The corn also provided a natural tax base.