The Proto-Celtic word *sextam
Word *sextam
Meaning seven
See also Avestan: hapta ‘seven, 7’

Other words that might be of interest
*oino ‘one’, *dwei ‘two’, *dwā(w) ‘two’, *swexs ‘6, six’, *nawan ‘nine, 9’, *tri ‘three, 3’, *kʷetru ‘four’, *kʷenkʷe ‘five, 5’