。
That is a large force numbering five thousand troops.
Literal
That [topic-は] 5000-person [gen-の] troops [obj-を] hold large-unit [copula-なのです].
擁する '(to) hold / possess / have under one's command' is a literary/formal verb reserved for large quantities or important holdings — troops, resources, supporters, territory. It carries a stately, weighty tone appropriate for military or political narrative. 兵員 'soldiers' is a formal collective noun. ~なのです is the explanatory form of the polite copula, giving the sentence a weighty formal presentation.