Im making a map that has an intro where you choose which race you want to be (custom races). I put all the players in Force 1 and used a "Display message" for an action. If there is more than one player in the force, will the trigger run more than once? Sorry for such a noobie question, but I can't test it right now. Also, I tried putting the triggers in "Current Player", but some triggers failed to run. Is there a better "place" to put the triggers? (there are four players by the way)

Give the triggers individually to each player, and if you can use "Current Player" in your triggers (its just convenient). Also, the trigger (in order to operate for a current player) must belong to the player.

Make sure that in your "forces" menu you have unchecked "Randomize start locations"

Triggers work like this:
Each player has their own copy of triggers. All Players and forces are translated into one trigger per player (so if you have an all players trigger that doesn't have a preserve trigger on it, it can run up to 8 times). First, Player 1 runs all his triggers in the order of the triggers, with any triggers from All Players or his force considered to be owned by him. Then Player 2 runs all his triggers in the same fashion, regardless of whether or not all of player 2's triggers are before player 1 in the trigger list. Repeat for the rest of the players.

In the case of "Display Message", it only works for current player, so if player 1 runs the display trigger first, then the action makes it so that the trigger won't run again, only player 1 will see the display text, and it won't run for the rest of the players. If the condition is true for all players, then it will be displayed up to 8 times in one trigger cycle, each on different players' computers (each individual person will only see the display text once).

The same is not true for other actions, like "create unit". If all 8 players run a create unit trigger, there will be 8 units, and everyone can see those 8 units.

The only legal/working owners of triggers are players 1-8, Force 1-4, and All Players. I'm unsure of the (non) allied victory players, but I doubt it. Note that conditions and actions can use the extra players just fine.

Nope. This is a "relative" player statement, each of the players has other players that they share allied victory with so there's no way for sc to determine what player to give it to and it won't even try.
When the map starts there's only players 1-8 as trigger owners because as you already explained the valid "wildcard" players (Forces, All Players) are being separated into specific players (P1-8).

Oh good! My main concern was "Display Message" being shown more than once. Thanks!