The Proto-Indo-European word *septḿ̥
Word *septḿ̥
Meaning seven
See also Homeric Greek: ἑπτά ‘seven’
Armenian: ewtᶜn ‘seven’
C. Luwian: ša-ap-ta-mi ‘seven’
Proto-Indo-European: *septm̥-mós ‘seventh’
Hittite: ša-ap-ta ‘seven’
Avestan: hapta ‘seven, 7’

Comments
Arm. evt'n/yot'/yot'ĕ, Alb. shtatë/shtatë, Lith. septyni, Ltv. septiņi, Gaul. sextan, Gm. sibun/sieben, Eng. seofon/seven, Gk. hepta, Av. hapta, Ir. secht/seacht, Hitt. šipta-, Kashmiri sath, Kamviri sut, Lat. septem, Oscan seften, ON sjau, Old Prussian septīnjai, Osset. avd/avd, Pers. /haft, OCS sedmĭ, Polish siedem, Russ. sem', Skr. sapta, Toch. ṣ pät/ṣ ukt, Goth. sibun, Welsh saith

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’, *septm̥-k̂ómt-h₂ ‘seventy’, *kswek̂s ‘six’, *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’, *kswek̂sos ‘sixth’, *kʷturyós~kʷetwortos ‘fourth’, *pn̥kʷtós ‘fifth’, *triyós ‘third’