Part of the ongoing Verse Quick Reference project.

Exo 4:8 “Then it will be, if they do not believe you, nor heed the message of the first sign, that they may believe the message of the latter sign.
Exo 4:9 And it shall be, if they do not believe even these two signs, or listen to your voice, that you shall take water from the river and pour it on the dry land. The water which you take from the river will become blood on the dry land.”

In Exodus 4, Moses argues against God that he should go to the people of Israel. In verse 1, Moses objects that the people will not believe that he is even sent from God. God gives Moses a miracle to perform “that they may believe that the LORD God of their fathers, the God of Abraham, the God of Isaac, and the God of Jacob, has appeared to you” (verse 4). But then God then gives Moses another sign.

This next sign is in case they do not believe the first sign. If they do not believe the first sign, Moses has a second sign. But then God gives a third sign, just in case they do not believe the first two.
This is a cascading contingency plan. This is hedging against the possibility of both plan A and plan B failing. God does not know how many signs it will take to make Israel believe, but He gives enough to cover what would be reasonable.