The English phrase "I will greatly mutliply" (A.V.) is translated from the Hebrew phrase הַרְבָּה אַרְבֶּה (harbah arbeh). This Hebrew phrase consists of two verbs, both in binyan Hif'il, the former being an infinitive absolute, while the latter is in the imperfect tense. This is a frequent Semiticism in the Hebrew Tanakh, and it should not be translated literally into English.
Regarding this particular construction, Wilhelm Gesenius wrote,1
(a) The infinitive absolute used before the verb to strengthen the
verbal idea, i.e. to emphasize in this way either the certainty
(especially in the case of threats) or the forcibleness and
completeness of an occurrence. In English, such an infinitive is
mostly expressed by a corresponding adverb, but sometimes merely by
putting greater stress on the verb; e.g. Gn 217 מוֹת
תָּמוּת thou shalt surely die, cf. 18:10, 18, 22:17, 28:22, 1 S
96 (cometh surely to pass); 24:21, Am 55,
717, Hb 23, Zc 1117; with the
infinitive strengthened by אַךְ Gn 4428 (but 27:30 and
Jacob was yet scarce gone out, &c.); Gn 433 הָעֵד הֵעִד
בָּ֫נוּ he did solemnly protest unto us; 1 S 206 נִשְׁאֹל
נִשְׁאַל David earnestly asked leave of me; Jos 1713, Ju
128 וְהוֹרֵישׁ לֹא הֽוֹרִישׁוֹ and did not utterly drive
them out; especially typical instances are Am 98 I will
destroy it from off the face of the earth אֶ֫פֶס כִּי לֹא הַשְׁמֵיד
אַשְׁמִיד וג׳ saving that I will not utterly destroy, &c.; Jer
3011 and will in no wise leave thee unpunished; cf. further
Gn 2018, 1 K 326, Jo 17, Jb
Again, it is used "to emphasize...the certainty (especially in the case of threats) or the forcibleness and completeness of an occurrence."
I believe the A.V. translates appropriately as emphasizing the magnitude of multiplying, hence, "I will greatly multiply..."
The English words "I will" are properly translated from the Hebrew verb אַרְבֶּה (arbeh) which is conjugated in 1st person ("I") and imperfect tense ("[I] will"). Think of the imperfect tense as English "future tense" in this case.
1 Wilhelm Gesenius. Gesenius' Hebrew Grammar, p. 342, §113n.