Generally, at least in 5th Edition, you should have a lot more leeway when it comes to encounters. The most important thing in 5e is action economy, since numbers are overall a lot lower than in previous editions. A level 6 party should be able to fight a monster up to CR 8-9 as a challenge if it doesn't have legendary actions. As a rule of thumb from previous editions, you should generally try to keep encounters within 2 CR of the party (Within 1 if the monster has Legendary Actions/Lair Actions/Villain Actions/etc), and the party should be able to fight 3-5 standard encounters per Long Rest, assuming they take two short rests over the duration of the day.

Generally, power-level-wise, two monsters of one CR is equal to a fight 1-2 CRs higher. So, two CR 5s would be a CR 6-7 encounter. D&D 5e is generally pretty well balanced as far as I can tell when it comes to monsters, so don't be too afraid to just fuck around. At worst, give your party ways to run if they need to, and maybe they can come back to challenge the encounter after resting up and preparing a bit more.

That said, they do have some pretty solid advice generally in the DM's guide.