21 δεῖ οὖν τῶν συνελθόντων ἡμῖν ἀνδρῶν
ἐν παντὶ χρόνῳ ᾧ εἰσῆλθεν καὶ ἐξῆλθεν
ἐφ’ ἡμᾶς ὁ Κύριος Ἰησοῦς,
22 ἀρξάμενος ἀπὸ τοῦ βαπτίσματος Ἰωάννου
ἕως τῆς ἡμέρας ἧς ἀνελήμφθη ἀφ’ ἡμῶν,
μάρτυρα τῆς ἀναστάσεως αὐτοῦ σὺν ἡμῖν
γενέσθαι ἕνα τούτων.
Here's a very short diagram of the initial bones of verse 21:
The 'something' is specified by the completing infinitive γενέσθαι, the aorist middle (deponent) infinitive of γίνομαι, 'I become'. Broadly translated, δεῖ . . . γενέσθαι means something like 'it is necessary (for someone or something) to become'.
But become what? And who is doing the becoming?
The subject of γενέσθαι is ἕνα, meaning 'one' (person). Ἕνα is the masculine singular accusative form for the number 'one'; accusative because it is the subject of an infinitive.
Ἕνα is modified by 'τούτων' ('of these') and - looking back to the beginning of verse 21 - 'τῶν συνελθόντων ἡμῖν ἀνδρῶν' - ('of the men who accompanied us').
So Peter is saying that 'one of the men who has been accompanying us', needs 'to become' - what?
21 Wherefore of these men which have companied with us all the time that the Lord Jesus went in and out among us,22 Beginning from the baptism of John, unto that same day that he was taken up from us, must one be ordained to be a witness with us of his resurrection.
21 Therefore it is necessary to choose one of the men who have been with us the whole time the Lord Jesus was living among us -Which one might think of as close enough to the same thing; but remember that the verb 'choose' is nowhere to be found in the Greek.