The question of how many voice cues a horse can reliably learn does not have a fixed ceiling — horses are capable of learning many more than the handful of basic gait cues that most handlers teach them. Research and practical experience from circus training, film horse training, liberty work, and clicker training programs have demonstrated that horses can learn dozens of specific cue-response associations when those cues are taught systematically and maintained with consistent reinforcement.
The limiting factor is not the horse's cognitive capacity but the handler's ability to create and maintain a consistent vocabulary. Each voice cue must be acoustically distinct from every other cue in the system — if walk and whoa sound too similar, the horse will confuse them. If good boy is spoken with the same tone as forward cues, the horse cannot tell whether it is being praised or asked to move. The more cues are added to the system, the more carefully each one must be differentiated from all the others.
Practically, most working horses are reliably trained to between five and fifteen voice cues — the basic gait transitions, a stop, a calming cue, a praise marker, and several specific requests relevant to their work. Liberty horses and horses trained with extensive clicker training often know considerably more. The key is that each cue is taught explicitly, tested regularly, and maintained through consistent use rather than assumed to be permanent after a few sessions of training.
Consistency across all handlers who work with the horse is also essential. A horse that knows ten voice cues from its trainer but receives no voice cues, or different voice cues, from other handlers who ride it will show variable responses that appear unreliable but actually reflect variable training input. Maintaining a written or communicated cue vocabulary for horses worked by multiple people is one of the practical habits that separates professionally managed horses from those whose training is inconsistent.