There are 12 normal endings and 1 bad ending to this game. They all depend on at what point you defeat Lavos and which method you use to get to him. Some endings you may need to get a New Game+, then beat the game.
Beyond Time
The initial ending. You can get this ending by reviving Crono at Death Peak and defeating Lavos. It may vary depending on whether you got Magus as a character or if you use the Epoch to get to Lavos.
Reunion
You can get this ending by defeating Lavos without reviving Crono at Death Peak. Just like "Beyond Time", this ending may vary if you used the Epoch or not.
The Dream Project
You can get this ending by defeating Lavos by going through the Right Telepod at the beginning of the game in New Game +. You can get this ending by killing Lavos in the Ocean Palace in New Game +.
The Successor of Guardia
The only way to get this ending is to kill Lavos by fighting him as soon as you return from the Middle Ages for the first time and before the Trial. Use the right Telepod.
Good Night
Fight Lavos after you reach the End of Time for the first time but before you fight at Zenan Bridge.
Legendary Hero
Kill Lavos after killing Zombor but before getting the Hero Medal from Tata.
The Unknown Past
Kill Lavos after getting the Hero Medal but before the Gate Key gets stolen.
People of the Times
Kill Lavos after getting back the Gate Key but before you give Frog the Masamune.
The Oath
Kill Lavos after giving the Masamune to Frog, but before fughting Magus.
Dino Age
Kill Lavos after you beat Magus but before you fight Azala.
What the Prophet Seeks...
Kill Lavos after you kill Azala but before Schala opens the magic seal in the Ocean Palace.
A Slide Show
Kill Lavos after opens the door to visit the Queen but don't power up Marle's Pendant.
Bad Ending
Get killed by Lavos anywhere except in the Ocean Palace.
