Though these are not closely parallel to the OP's example, I thought it might be instructive to look for a few examples of the usage of amplius. The first one also uses pluris (=plures). Perhaps no. 2 is most relevant.
1. Sallust, B.I., 80.6.1 :
verum ea necessitudo apud Numidas Maurosque levis ducitur, quia singuli pro opibus quisque quam plurumas uxores, denas alii, alii pluris habent, sed reges eo amplius.
But such a tie is taken lightly among the Numidians and Moors because individuals have as many wives as their means permit—some as many as ten, others more, and kings a still greater number. (Loeb trans.)
2. Pliny, Nat. Hist. 8.133.1
Praeparant hiemi et irenacei cibos ac volutati supra iacentia poma adfixa spinis, unum amplius tenentes ore, portant in cavas arbores.
Hedgehogs also prepare food for winter, and fixing fallen apples on their spines by rolling on them and holding one more in their mouth carry them to hollow trees. (Loeb)
3. Livy, AUC 36.38.3
duas amplius horas dubium certamen sustinuere;
and for more than two hours they kept the outcome of the battle in doubt. (Loeb)
4. Livy, AUC 37.24.6
Cum in alto, ubi substiterant, cibo reficerent vires, contemplatus Eudamus hostes claudas mutilatasque naves apertis navibus remulco trahentes, viginti paulo amplius integras abscedentes,
While they were rebuilding their strength with food where they had stopped on the open sea, Eudamus watched the enemy hauling their disabled and damaged vessels with tow ropes from open-decked ships, not many more than twenty moving off unharmed. (Loeb)
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