proper_divisors(n) := disjoin(n, divisors(n))$
for i from 1 thru 10 do sprint(i, ":", proper_divisors(i))$

pdivcount(n) := cardinality(proper_divisors(n))$
block(
    maxdivs: 1, nmax: 1,
    for i from 1 thru 20000 do(
        divs: pdivcount(i),
        if divs > maxdivs then block(
            maxdivs: divs,
            nmax: i
        )
    ),
    sprint(nmax, "has", maxdivs, "proper divisors.")
)$