Oh, its doesn't get replenished like the unit offer does? You just add the amount to the existing ones?
When you start a new round, it does get replenished like the unit offer does. But when you start a new round, the existing units and AAs in the unit offer do not stay there.
When you start a new round, all unrecruited units in the unit offer go on the bottom of their silver (or gold) unit decks, and the unit offer is replenished by bringing out the number of new units equal to the number of players plus two. All unlearned AAs in the unit offer go on the bottom of the AA deck, and bring out into the unit offer the number of new AAs equal to the number of unburnt monasteries.
AAs in the AA offer and spells in the spell offer are replenished in a different manner.
The only difference is that when a tile containing a monastery is explored, an additional AA enters the unit offer, but no new units. In this case, the existing AAs in the unit offer stay there.