As usual, the Professor is correct, and I am currently playing the "more hordes" mod on the "more regions" map.
I prudently let the Romano British faction in emergent and thus non-playable status.
The two modifications go along quite well.
I also raised the max number units in hordes and lowered the minimum number, that makes the importance of the starting settlement more meaningful, no more huge numbers pouring out of a depopulated hamlet.
I am also thinking of reducing the time needed to produce some units, I feel that people able to become instantly a horde should not have to wait 5 years to muster 10 units of spearmen.