2 syllables:
blunder, bumper, bunker, clumber, cumber, dumper, dunker, hunger, jumper, junker, lumber, lumper, monger, number, plunder, plunker, slumber, stumper, sunder, thunder, tunker, umber, unbar, under, wonder, younger, younker

3 syllables:
seersucker, foregather, mortgagor, worshiper, armature, orbiter, curvature, archosaur, formalwear, sharksucker, merganser, sarcomere, earthenware, warmed-over, whoremonger, harvester, germander, gardener, sharpener, forfeiture, purchaser, carpenter, arthromere, cornhusker, worshipper, pardoner, hurdler, porte-cochere, circular, foremother, vers libre, hardcover, arthrospore, asunder, back-number, bob under, box-number, boy wonder, call number, encumber, fall under, f number, go under, large number, like thunder, mach number, mass number, outnumber, page number, phone number, pole jumper, prime number, put under, real number, ski jumper, spelunker, stop number, tub-thumper, wave number, whole number

4 syllables:
determiner, interpreter, conservator, reversioner, extortioner, conservatoire, buckle under, complex number, compound number, disencumber, going under, index number, knuckle under, license number, magic number, newtown wonder, octane number, power hunger, po box number

5 syllables:
landscape gardener, spinal curvature, atomic number, cardinal number, kentucky wonder, natural number, opposite number, parachute jumper, rational number, telephone number

6 syllables:
center of curvature, circle of curvature, algebraic number, irrational number, registration number, transcendental number

7 syllables:
radius of curvature, imaginary number

8 syllables:
identification number, pure imaginary number, social security number

9 syllables:
bank identification number

12 syllables:
imaginary part of a complex number