TEXT: Simple 5e Encounter Building Guidelines
  T
  hese guidelines are intended to help dungeon
  masters quickly build combat encounters for
  5e. These guidelines are built to show dungeon
  masters what a "hard" encounter looks like so
  you have the freedom to tune the battle to
  make it easier or harder depending on the
  story and your goals for the encounter.
  Because many variables can change the actual difficulty of
  encounter, consider these loose guidelines, not hard rules.
  Four Steps for Building
  Combat Encounters
      1. Start with the story. What type of encounter makes
  sense for the current location and direction of this story? You
  might know this ahead of time or might want to build a quick
  encounter right at the table. When designing an encounter,
  start with the story first and the mechanics second.
  2. Choose appropriate monsters. Use the monsters by
  challenge rating index or the excellent Monsters by
  Environment list in appendix B of the Dungeon Master's
  Guide to help you choose the right monsters for the
  encounter you want to build. Aim for creatures with a
  challenge rating roughly equivalent to 3/4 the level of the
  characters or less. If you're aiming to have them fight a single
  monster, the challenge rating should be at the character's
  level or up to two ratings higher.
  3. Determine the number of monsters. Choose a number
  of monsters based on the monsters' challenge rating
  compared to the characters' level.
  For monsters with a challenge rating of 1/4 the character's
  level, use two monsters per character.
  For monsters with a challenge rating of 1/3 the character's
  level, use one monster per character.
  For monsters with a challenge rating of 3/4 the character's
  level, use one monster per two characters.
  For monsters with a challenge rating equal to or above the
  character's level, use one monster per four characters.
  4. Adjust difficulty. These guidelines aim for a "hard"
  encounter as described on page 82 of the Dungeon Master's
  Guide. Increase or decrease the difficulty by adding or
  subtracting monsters, choosing monsters with higher or
  lower challenge ratings, or by increasing or decreasing the
  monsters' hit points.
  5. Evaluate the action economy. Compare the number of
  monsters to the number of characters. Too many creatures on
  one side or the other will have a big effect on the difficulty.
  Big monsters that fight by themselves are much easier to
  defeat than a somewhat big monsters with a couple of lackeys
  to keep the wizards busy. In general, for most encounters,
  have at least one monster for the first four characters plus
  one monster for each additional character.
  Quick Lookup Table
  The table below helps you select the appropriate challenge
  rating for monsters based on the number of monsters per
  character at a certain character level. These too are based on
  a "hard" encounter as outlined in the Dungeon Master's
  Guide. This table is slightly more accurate than the rules of
  thumb in part 3.
  Character
  level
  CR for two
  monsters
  per
  character
  CR for one
  monster
  per
  character
  CR for one
  monster per
  two
  characters
  CR for one
  monster per
  four
  characters
  1
  Too hard
  1/4
  1/2
  1
  2
  1/8
  1/4
  1
  3
  3
  1/4
  1
  2
  4
  4
  1/2
  1
  3
  5
  5
  1
  2
  4
  6
  6
  1
  2
  5
  7
  7
  1
  3
  5
  8
  8
  2
  3
  6
  9
  9
  2
  4
  7
  10
  10
  2
  4
  7
  11
  11
  3
  5
  8
  12
  12
  3
  5
  9
  13
  13
  3
  5
  10
  14
  14
  4
  6
  11
  15
  15
  4
  6
  11
  16
  16
  4
  7
  12
  17
  17
  4
  8
  13
  18
  18
  5
  8
  14
  19
  19
  5
  9
  15
  20
  20
  6
  10
  16
  21
  Loose Guidelines, not
  Hard Rules
  The challenge of any combat encounter cannot be easily
  measured or quantified. Too many variables can increase or
  decrease the difficulty of a battle. These include the variance
  in difficulty between monsters at the same challenge rating,
  the class mixture of characters, the particular spells a
  character may have, which side wins initiative, how many
  resources the characters have exhausted previously,
  circumstantial or environmental considerations, and the
  skills and proficiency of the players
  All encounter building systems, including this one, lack
  precision. Thus, we aim for faster, looser guidelines to help
  dungeon master's quickly build encounters while recognizing
  that actual difficulty will still vary.
  
SOURCE: textdata/thevault/Dungeons & Dragons [multi]/5th Edition (5e)/5e_encounter_building_guidelines.pdf