John Gill Commentary Numbers 3:46

"And for the redemption of the two hundred and threescore and thirteen of the first-born of the children of Israel, that are over and above [the number of] the Levites," — Numbers 3:46 (ASV)

And for those that are to be redeemed
With money, there being not Levites enough to answer to them, and exchange for them:

of the two hundred and threescore and thirteen of the firstborn of the
children of Israel, which are more than the Levites ;
for the sum total of the Levites, as given, was but 22,000, (Numbers 3:39) , whereas the sum total of the firstborn of Israel were 22,273, (Numbers 3:43) , so that there were, two hundred seventy three more of the latter than of the former; and what was to be done with these next follows.