This question already has an answer here:
Let's say I have an adventure, involving four 4-level heroes.
From the CR description I can assume that one CR 4 monster would be a decent challenge for them, in terms of a combat encounter. Can one CR 4 monster be replaced with four CR 1 monsters or two CR 2 monsters? (I guess it doesn't work like that, it is just an example)
And what if my party has only 3 players (and 3 heroes), how can I scale down the encounter? Should it be a CR 3 monster, or (respectively) three CR 1 monsters?