The Proto-Indo-European word *kswek̂s
Word *kswek̂s
Meaning six
See also Homeric Greek: ἕξ ‘six’
Early Proto-Albanian: *seksti ‘six’
Proto-Indo-European: *kswek̂sos ‘sixth’
Avestan: xšuuaš ‘six, 6’

Comments
Arm. vec'/vec'/vec', Alb. gjashtë, Lith. šeši, Ltv. seši, Gaul. suex, Gm. sēhs/sechs, Eng. siex/six, Gk. heks, Av. xš.uuaš, Illyr. ses-, Ir. sé/sé, Kashmiri śe, Kamviri ṣ u, Lat. sex, Oscan sehs, ON sex, Old Prussian usjai, Osset. æxsæz/æxsæz, Pers. /šeš, OCS šestĭ, Polish sześć, Russ. šest', Skr. ṣ aṣ , Toch. ṣ äk/ṣ kas, Umbrian sehs, Goth. saihs, Welsh chwech

Other words that might be of interest
*hₓok̂tṓ(u) ‘eight’, *trei- ‘three’, *dwoi- ‘group of two, two’, *h₁oinos ‘one’, *kʷetwóres (m), kʷetwesóres (f), kʷetwōr (n) ‘four’, *(h₁)néwn̥ ‘nine’, *pénkʷe ‘five’, *sem- ‘one’, *septḿ̥ ‘seven’, *septm̥-k̂ómt-h₂ ‘seventy’, *tréies (m), trisóres (f), treih₂ (n) tri- [prefix] ‘three’, *dwéh₃(u) ‘two’, *h₁newh₁m̥(h₁néwh₁n̥?) ‘nine’, *tréyes ‘three’, *trī-k̂omt(hₐ) ‘thirty’, *kswek̂s-k̂omt(hₐ) ‘sixty’, *kʷetwóres ‘four’, *per(hₓ)- ‘first’, *dwiyos~dwitos ‘second’, *h₁newh₁m̥m/n̥-mos ‘ninth’, *hₓok̂towós ‘eighth’, *kʷturyós~kʷetwortos ‘fourth’, *pn̥kʷtós ‘fifth’, *septm̥-mós ‘seventh’, *triyós ‘third’