were. Supply Ellipsis, " they found", instead of "were". Compare Numbers 21:16-18. The scene at Rephidim (Exodus 17) after the thirty-nine years was be-cause they expected God to provide for them still.
twelve... threescore... ten. See App-10.
Related Commentaries of Exodus 15:27
Exodus 15:27
27 And they came to Elim, where were twelve wells of water, and threescore and ten palm trees: and they encamped there by the waters.