Hi, i don't have much experience as a dm, and encounter building, here's my questions:
The target encounter for x pcs of y level, should be distributed trough a whole dungeon, or various encounters per dungeon?
how many encounters is too much? before a short rest, and a extended rest?
how many encounters is too much before a really hard encounter?
thanks in advance.
The DMG guidelines are pretty good, but if you need a relatievely sure-fire way of determining whether your party can do "one more battle", then simply look at the number of healing surges they have remaining.
To answer your questions individually however:
The "encounter budget" listed on page 57 of the DMG tells you how much experience you can "spend" in an encounter, without making it particularly under or overwhelming. For example, it's recommended that a group of four level 5 PCs should fight challenges that add up to "roughly" 800XP. That's equivalent to four level 5 "standard" monsters. You can spend this "XP" as you wish, either on monsters of various roles and levels, or on traps and hazards (DMG pg 85).
Keep in mind that an "equal level" encounter won't sap many of a party's resources, barring bad luck; you can quite safely spend more "XP" without worrying too much whether your party can handle it. As a rough guide, don't be afraid to spend up to (or even a little past) the XP budget that would be used if adding "one more PC". For example, the XP budget for four level 5 PCs is 800, but you can spend as if there were "five" PCs (i.e. 1000 xp) without much concern. Spending more than that might cause an encounter to drag however.
The system assumes that the PCs will get into four combats, with a short rest in between them, and an extended rest afterwards, but keep in mind that the only "expendable" resources that PCs need to worry about are healing surges and Daily powers. In general they'll tell you if they want to "extended rest" or not.
As there is a rough limit to the number of surges a player can spend in combat (about three; one second wind, one bump from a leader class, and maybe being able to spend a surge from a potion or magic item), use that as a guide to determine whether the PCs "need" to rest or not. A more difficult encounter isn't going to increase their ability to "heal" themselves more, so focus on that rather than on how many arbitrary fights they do beforehand.
What I recommend is to spend the first level or two playing the system "by the book"; see if your PCs are finding the results challenging enough, and how many encounters they get through before "wanting" to rest. You'll quickly get a feel for what your party can/cannot handle.
Another thing to be careful with (especially at low levels) is the maximum damage the monsters/traps/whatever can do.
If your level 1 character has 24 hit points and the 'level appropriate' early encounter can do 19 points of damage with a single attack... you're gonna have a bad day. (True story)
At level 8, I'm finding (again from published WotC modules), that 'level' appropriate stuff is doing more than 1/3rd of my hit points in damage per hit. This gives me a slightly bigger buffer (it takes 3 hits to KO instead of 2) but it still feels very dangerous.
Even with Warlord or Cleric assistance there's an interesting problem, monsters hit me for more than my healing surge value, so if I second wind it effectively means a wasted turn if the monster hits me the turn I second winded. Even with being Worded by the leader, the monsters still hit for more than the surge+extras... and the Leader only has two words to give out per combat.
What if I told you that there is a list of True Dragons, and Dragonwrought Kobolds are not on that list?
What if I told you the words 'planar traits' never occur in the RAW of the Genesis spell?
What if I told you Versatile Spellcaster explicitly only allows you to cast spells you already know?
When choosing how to interpret RAW you should always select the definition that does the least injury to the rest of the rules.
The thing that's hard to come up with, even once you've balanced encounters, is that often times, it's rather un-immersive to rest. That's to say; when you're clearing the catacombs of an undead menace, there's not a very large chance your players are gonna thing: Hey! let's drop for 8 hours ang keep this thing going in the morning.
The things all theese monsters could come up with if you give them 8 hours. Whoh-ha!
in other words: the main issue for me, personally, is to not make the series of encounters too long to too draining that the characters will need an extended rest before they're out of any emidiate danger. Or rather, untill there's a time when a player (and their characters) would actually consider setting camp.
You ever punched a polarbear for making you miss the buss? It's fun.