No time for a discussion right now, but I would think myself that these Orcs were acting under the power (that came from his continuing presence in the world rather than direct control, obviously) of Sauron rather than Saruman. Saruman would have gathered them and made use of them, though, and this would be the stock from which he built up his large armies.
20,000 men isn't much... a full mobilisation I would suspect to be rather more, but of course the prowess of the fierce Rohirrim was formidable as we know.
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