What does vīgintī ūnus mean in Latin?

vīgintī ūnus, a numeral, means “twenty-one; 21”.

How do you pronounce vīgintī ūnus?

In classical Latin, [wiːˈɡɪn.tiː ˈuː.nʊs]; in ecclesiastical (church) Latin, [viˈd͡ʒin.ti ˈuː.nus].

numeralindeclinable portion with a first/second-declension numeral, singular only

vīgintī ūnus (feminine vīgintī ūna, neuter vīgintī ūnum)

Classical[wiːˈɡɪn.tiː ˈuː.nʊs]Ecclesiastical[viˈd͡ʒin.ti ˈuː.nus]

singularsingular onlytwenty-one; 21
elephanti viginti unustwenty-one elephants27 BCE – 25 BCE, Titus Livius, Ab urbe condita 21.22.4

singular
form	masculine	feminine	neuter
nominative	vīgintī ūnus	vīgintī ūna	vīgintī ūnum
accusative	vīgintī ūnum	vīgintī ūnam	vīgintī ūnum
ablative	vīgintī ūnō	vīgintī ūnā	vīgintī ūnō
vocative	vīgintī ūne	vīgintī ūna	vīgintī ūnum

From vīgintī (“twenty”) + ūnus (“one”).