I'm reading through the new DM D&D Basic Rules, but I'm confused about the challenge rating of a monster.
Does the challenge rating assume a single PC or a group of PC's? As a CR3 a challenge for 1 PC (like with 4th Edition), or for a group of PC's? If so, for how large a group? How should a DM choose monster with more or less PC's?
Neither and both.
Unlike in Previous editions, CR isn't used to directly create combat encounters, instead the XP values of the creatures ( which is tied directly to CR) is used to determine how many creatures can be used in the encounter. Instead, CR tells you the upper maximum difficulty of the monster, assuming a party of 4. One of the reasons why they moved away from using Level instead of CR, is so that you did not confuse the CR of the Monster with the level of the characters fighting the monster.
So, basically a CR1 monser means that if your characters are level 1, this will be a challenge for them. If they are level 2, it will be pretty easy depending on how you use the xp budget to put 1 or more monsters into the battle. If the monster is CR 1/4, then that means that if you have a single character alone, without the party it will be a challenge for them, 1/8 means 2 will be a challenge for an indvidual, and a CR 0 means it's not a challenge for anybody. You will notice that CR 1/8th has an XP value of 25 while a CR 0 has an xp value of 10.
But the important criteria you want to use when buliding an encounter is your XP budget. Use CR to only filter out which creatures are too deadly for the level of your party of any size.
Challenge Rating, page 56
Much of the advice in this section focuses on the XP values of monsters and encounters, as opposed to their challenge rating. Challenge rating is only a guidepost that indicates at what level that monster becomes an appropriate challenge. When putting together an encounter or adventure, especially at lower levels, exercise caution when using monsters whose challenge rating is higher than the party’s level. Such a creature might deal enough damage with a single action to overwhelm PCs of a lower level. Even though an ogre has a challenge rating of 2, for example, it can kill a 1st-level wizard or sorcerer outright with a single blow.
When building encounters, use XP values. Those XP values along with the relevant multipliers for party size and number of monsters, are what are important when determining how deadly a monster is to the party or to a character. CR is only a filter for the "upper limit" of which monsters you should choose from when using the XP budgets.
So in the end, trying to understand how many party members a CR is coordinated against, becomes a bit of a red herring for when it matters. The answer is "4", but that isn't really the answer you want, what you want to focus on is your XP budget, and the multipliers for party size and number of monsters.
A single monster will present a medium or hard challenge against a party of four PCs of a level equal to its CR. (Page 56, "Simple Encounters") Choosing monsters for larger or smaller parties is covered on page 58, "Larger or Smaller Parties".
