By a multiple relation I mean a relation which in any concrete instance of its occurrence necessarily involves more than two relata.

For example, when John likes Thomas there are only two relata, John and Thomas.

But when John gives that book to Thomas there are three relata, John, that book, and Thomas.
Some schools of philosophy, under the influence of the Aristotelian logic and the Aristotelian philosophy, endeavour to get on without admitting any relations at all except that of substance and attribute.
Namely all apparent relations are to be resolvable into the concurrent existence of substances with contrasted attributes.

It is fairly obvious that the Leibnizian monadology is the necessary outcome of any such philosophy.

If you dislike pluralism, there will be only one monad.
Other schools of philosophy admit relations but obstinately refuse to contemplate relations with more than two relata.