1 syllable:
-plex, aix, beck's, becks, blech's, checks, cheques, czechs, de-sex, dec's, decks, dex, eckes, eques, ex, fecks, flecks, flex, flex-, fss ecs, heck's, hex, jex, kecks, klecks, lex, lzexe, meckes, necks, nex, next, pdftex, pechs, peck's, pecks, plex, recks, rex, rtecs, sechs, secks, sekes, sex, sheck's, shek's, snecks, specks, specs, sphex, sveccs, tech's, techs, teck's, tex, thraex, treks, vex, wrecks, x, x.

2 syllables:
aflex, alecs, altex, amex, ampex, annex, annexe, apec's, apecs, apex, apmex, armtek's, artex, asecs, astex, auspex, autex, avtex, aztecs, bedecks, berlex, bibtex, binhex, bitmex, bomex, bowflex, cache-sexe, caltex, camex, capex, carex, carmex, cedex, celtx, cemex, centex, cimex, cmx, codecs, codex, colmex, comex, compex, complex, complexe, comtrex, connex, convex, convexe, copecks, corex, cplex, craptex, cromlechs, crooknecks, crosschecks, crownx, cryptex, cusecs, daplex, datex, defex, deflex, dentex, desex, devex, digex, diplex, dissects, duplex, effects, ejects, endex, entex, execs, expects, eyetech's, fanech's, fedex, finex, firex, fourplex, frontex, frutex, funplex, futex, gatx, gentex, gitex, gore-tex, goretex, graflex, gtech's, hapex, have sex, henpecks, hiprex, imbrex, implex, inflex, inmex, inpex, insex, intex, invecs, jadetex, katex, keflex, klaypex, knipex, komplex, konvex, kopecks, kotex, latex, lipex, logex, longnecks, mapex, memeplex, memex, memtec's, metex, mi-sex, miktex, mindflex, mixtecs, mutex, n.mex, narthex, natec's, navtex, nobec's, noindex, nomex, norflex, nuttx, nymex, nynex, objects, onex, opec's, opecs, parex, parsecs, paychecks, paychex, pemex, perplex, perspex, pewex, phanteks, phone-sex, pictex, pipex, playtex, podex, pokedex, portec's, pretex, proflex, projects, q-flex, qintex, quadplex, quebec's, railtex, ramex, re-flex, re/flex, rebecks, rebecs, rechecks, redex, rednecks, rednex, reflects, rejects, remex, respects, retex, romex, safe-sex, safe sex, same-sex, sarex, scitex, screenx, sensex, sharex, silvex, simplex, sofex, spot-checks, starplex, startex, steinbeck's, subjects, subutex, suplex, supplex, suspects, synflex, syntex, sysex, sysplex, tarex, telex, telmex, tenrecs, tex-mex, texmex, timeplex, tmp-smx, toltecs, torpex, transex, tremex, tridex, trihex, trintex, triplex, truex, twistex, typex, unsex, vertex, videx, vieques, visx, wall-tex, wedtech's, westtech's, winhex, winmx, wrynecks, xetex, yu-mex, zebecks

3 syllables:
5-simplex, adaplex, adaptec's, aeroflex, afterdecks, alopex, ametek's, ams-latex, andratx, aniplex, antapex, apoplex, apotex, aramex, ark-la-tex, artifex, aruspex, atriplex, atx, banamex, belowdecks, between decks, biconvex, biotechs, bottlenecks, celotex, cineplex, circonflexe, circumflex, clarinex, computex, contraplex, cybersex, cytotechs, datarex, decomplex, deus ex, discotheques, dorsiflex, double-checks, echoplex, endosex, equitex, erosstx, eurochecks, fantomex, faslodex, ferromex, flexatx, foredecks, full-duplex, g-quadruplex, gendrisek's, genentech's, glendimplex, googleplex, googolplex, goosenecks, govpx, half-duplex, haruspex, homosex, honda nsx, hudson & rex, humidex, hyperflex, hyponex, inditex, intellects, intel xe, interrex, intersects, intersex, intertex, iphone x, kaisertech's, keepassx, leathernecks, leibniz-keks, leicaflex, liga mx, lil nas x, log-convex, luatex, macosx, malcolm x, medarex, mediplex, megaplex, memorex, methanex, metroplex, middlesex, mineplex, mini-stx, mirapex, movieplex, multidex, multiplex, neorx, notimex, novamex, oeko-tex, oldcodex, oraflex, oral sex, orbitx, panorex, photodex, pillowtex, prentnieks, quadraplex, quadruplex, quarterdecks, quinidex, retroflex, rolleiflex, rolodex, rubbernecks, single-sex, sonic x, stratoflex, suffrutex, supaplex, supermex, superx, symantec's, teleflex, teletex, turtlenecks, tussionex, umarex, unisex, unperplex, ventritex, vert.x, zanaflex, zoladex

4 syllables:
adaptaplex, ameritech's, arimidex, audiotex, begonia rex, cinematheques, citeseerx, cmd.exe, ddx3x, demultiplex, directx, double-duplex, euroduplex, herpes simplex, libvpx, masters of sex, megaparsecs, mini-itx, nano-itx, nasa-tlx, osiris-rex, pico-itx, plano-convex, planoconvex, rejctx, single-duplex, solar apex, sugammadex, terminatexe, tokyo mx, tracey lambrechs, videotex, waifu2x

5 syllables:
charli xcx, concavo-convex, convexo-convex, dvb-s2x, exo-cbx, freestyle bmx, gaztelugatxe, goatse.cx, heteroduplex, tesla model x

6 syllables:
cigarettes after sex, extramarital sex, final fantasy x, nissan 240sx, non-penetrative sex, tyrannosaurus rex, unsimulated sex

7 syllables:
regression of y on x

8 syllables:
american history x