John Gill Commentary Numbers 11:21

"And Moses said, The people, among whom I am, are six hundred thousand footmen; and thou hast said, I will give them flesh, that they may eat a whole month." — Numbers 11:21 (ASV)

And Moses said
By way of objection to what God had promised, distrusting his power to perform:

the people amongst whom I [am] ;
among whom he dwelt, of whom he was a part, and over whom he was a ruler:

[are] six hundred thousand footmen ;
that were able to travel on foot, and were fit for war: this was the number of them when they came out of Egypt, (Exodus 12:37) ; they amounted in their last numbering to 3,550 more, which lesser number is here omitted, as Aben Ezra and Jarchi observe, and only the round number given: some say that all above the six hundred thousand were destroyed by the fire at Taberah, (Numbers 11:1) ;

and you have said, one will give them flesh, that they may eat a
whole month ;
this Moses could not tell how to credit.