However the strata manager presents things, you (IE the OC) either agrees to the contract or it doesn’t and goes elsewhere.
It does make some sense to calculate a fee based partly on the number of units. Some things a managing agent does would be fixed tasks and the same amount of work whether there are 10 units or 100. Some other tasks might be expected to increase with the number of units. So, a base fee plus a per unit fee does not seem unreasonable to me.
PS. Welcome ‘Strataact’, another from the ACT.