# Your descendants would have been as numerous as the sand, and the offspring from your womb as numerous as the grains of sand

These both mean that the people would have had more descendants than they could count. 

# the offspring from your womb

Yahweh speaks of the descendants of the people of Israel as if they were children to which the nation gives birth. 

# their name would not have been cut off nor blotted out

"I would not have destroyed them" 

# their name

"they"