וַיֹּ֤אמֶר לָהּ֙ מַלְאַ֣ךְ יְהוָ֔ה הַרְבָּ֥ה אַרְבֶּ֖ה אֶת־זַרְעֵ֑ךְ וְלֹ֥א יִסָּפֵ֖ר מֵרֹֽב׃
Díjole también el ángel de SEÑOR: Multiplicaré tanto tu linaje, que no será contado á causa de la muchedumbre.
Radak
הרבה ארבה, the angel being G'd's messenger, may speak in the first person as if he were to do what he says, seeing that he delivers G'd's message. It is as if G'd Himself had spoken.
Rav Hirsch
Um diesen Preis sollst du Stammmutter einer zahlreichen Nachkommenschaft werden. Hagar rührt sich nicht.
Radak
ולא יספר מרב, the words: "and it will not be capable of being counted due to its large number," are to be understood as an exaggeration, poetical license.