It doesn't say that unfortunately. It's an anacoluthon, which says: "Caesar's veterans come together in greater numbers every day, but who of you would dare to tell them face to face that by whom lands must be given back to the republic (or "lands of the republic must be given back"), since the acts of the man who donated them have now been revoked". Clearly the a quibus and his do not sit happily together.Caesar's veterans arrive more numerous each day, but who of you would dare to tell them face to face that lands must be given back by them to the republic, since the acts of the man who offered them (these lands) have now been revoked.
crebriores in dies conveniunt Caesaris veterani, a quibus terras rei publicae reddendas esse, cum abrogata iam sint illius acta qui has donaverit, quis vestrum palam dicere audeat? would make better sense. There are further ways of making sense out of it, but, given the faulty nature of this speech generally, I don't think the exercise is worth the candle really.