The replacement effects (As [this permanent] enters the battlefield, You may have [this permanent] enter the battlefield as..., and [This permanent] enters the battlefield with) will look at the state of the battlefield before the group of permanents enters.
The triggered abilities (When [a permanent with certain characteristics] enters the battlefield) will trigger for all eligible permanents entering the battlefield. This is true even if the ability is on one of the permanents that is entering the battlefield.
Examples
If you use Scapeshift to find several copies of Valakut, the Molten Pinnacle, some mountains, and several copies of Vesuva, you will have a group of lands entering the battlefield simultaneously. Vesuva's ability is a replacement effect, and so can only copy lands on the battlefield before the group of lands enters (so if you sacrificed all of your lands to Scapeshift, Vesuva would only be able to copy an opponent's land). Meanwhile, Valakut has a triggered ability. All of the Valakut abilities will trigger for all of the mountains entering the battlefield. If the Valakuts happen to be mountains as well, such as due to Prismatic Omen, they will trigger their own and each others abilities.
If you use Living Death, any instances of Clone that come back to life will have no valid copy targets and Canker Abomination will be a 6/6, since both of these are replacement effects. Meanwhile, Goldnight Commander has a triggered ability, and so will give all of your creatures +1/+1 until end of turn for each creature besides himself that entered the battlefield.
Rules Justification
The behavior of the triggered abilities comes from the following core rules:
603.6a Enters-the-battlefield abilities trigger when a permanent enters the battlefield. These are written, "When [this object] enters
the battlefield, ..." or "Whenever a [type] enters the battlefield,
..." Each time an event puts one or more permanents onto the
battlefield, all permanents on the battlefield (including the
newcomers) are checked for any enters-the-battlefield triggers that
match the event.
603.3b If multiple abilities have triggered since the last time a player received priority, each player, in APNAP order, puts triggered
abilities he or she controls on the stack in any order he or she
chooses. (See rule 101.4.) Then the game once again checks for and
resolves state-based actions until none are performed, then abilities
that triggered during this process go on the stack. This process
repeats until no new state-based actions are performed and no
abilities trigger. Then the appropriate player gets priority.
The behavior of the replacement effects comes from the following core rules:
608.2g If an effect requires information from the game (such as the number of creatures on the battlefield), the answer is determined only
once, when the effect is applied
614.1c Effects that read "[This permanent] enters the battlefield with ... ," "As [this permanent] enters the battlefield ... ," or
"[This permanent] enters the battlefield as ... " are replacement
effects.
614.12. Some replacement effects modify how a permanent enters the battlefield. (See rules 614.1c–d.) Such effects may come from the
permanent itself if they affect only that permanent (as opposed to a
general subset of permanents that includes it). They may also come
from other sources. To determine which replacement effects apply and
how they apply, check the characteristics of the permanent as it would
exist on the battlefield, taking into account replacement effects that
have already modified how it enters the battlefield (see rule 616.1),
continuous effects generated by the resolution of spells or abilities
that changed the permanent’s characteristics on the stack (see rule
400.7a), and continuous effects from the permanent’s own static abilities, but ignoring continuous effects from any other source that
would affect it.
614.12a If a replacement effect that modifies how a permanent enters the battlefield requires a choice, that choice is made before the
permanent enters the battlefield.
614.4. Replacement effects must exist before the appropriate event occurs—they can’t “go back in time” and change something that’s
already happened. Spells or abilities that generate these effects are
often cast or activated in response to whatever would produce the
event and thus resolve before that event would occur.
101.4. If multiple players would make choices and/or take actions at the same time, the active player (the player whose turn it is) makes
any choices required, then the next player in turn order (usually the
player seated to the active player’s left) makes any choices required,
followed by the remaining nonactive players in turn order. Then the
actions happen simultaneously. This rule is often referred to as the
“Active Player, Nonactive Player (APNAP) order” rule.
