John Gill Commentary Genesis 16:10

"And the angel of Jehovah said unto her, I will greatly multiply thy seed, that it shall not be numbered for multitude." — Genesis 16:10 (ASV)

And the angel of the Lord said to her
The same as before, who, by what follows, appears to be Jehovah himself:

I will multiply your seed exceedingly ;
not that she should have many children herself, for that she had more than this one she now went with, is not certain; but that the seed she had conceived should be exceedingly multiplied, and he should have a numerous posterity, as he had twelve princes sprung from him, the heads of Arab nations:

that it shall not be numbered for multitude ;
such the Turks are at this day, supposed to be the seed of Ishmael, Hagar's son.