It isn’t a basing rule, per se, but there is one rule that could act as a practical limitation on basing. Dragon Rampant has a unit cohesion rule, which requires that all the figures/bases in a unit be within 3 inches of one centrally located figure/base. That central figure isn’t necessarily a “leader” - it could be any figure and could change from turn to turn (although using a standard bearer or other distinctive figure would look cool). That central figure is that turn’s reference point to judge cohesion for the ‘blob’ of figures in the unit.
In practice, the cohesion rule means that all figures must have part of their base touching or inside a circle approximately 7 inches in diameter (assuming the figures are on 1inch bases). Normally it not a problem to fit twelve 28mm figures that are single-based in cohesion. If the bases are larger, though, it could be awkward getting 12 inside the cohesion zone (ex, if using a 10mm army based on 60x30mm rectangles, trying to get 12 bases in cohesion might be a challenge…).
I only mention the cohesion rule in case your planned armies are way outside the usual! Even then, reasonable players should be able to find a work-around that will allow using nearly any collection. (Ex, for the 12x 60x30mm bases, house rule that they are in cohesion when deployed in base to base contact 6 wide by 2 deep, even though the bases on the outside of the formation would be more than 3 inches from a base in the middle).