Can someone tell me how many characters/people am I suppose to duel?

There are 22 duelists in Worldwide Edition. Here is a list of all of them and howto get them.

These are the ones that you don't have to do anything to get:

Yugi Motou
Tea Gardner
Joey Wheeler
Ryou Bakura
Seto Kaiba
Ishizu Ishtar
Rex Raptor
Weevil Underwood
Espa Roba
Trusdale/Solomon (Yugi's Grandfather)
Mako Tsunami
Mai Valentine

Sometimes you may receive a message saying that there have been Ghoul Appearances, and you should watch out.

The following people people may appear during the week the Ghouls around.

You might find them when you go to talk to a passenger, or they may jump in when you are about to duel one of the regular duelists. However, you will only find one 1-3 times during that week.

But there will be other weeks when the Ghouls are around. If you defeat them, you will receive 5 rare cards, instead of you choosing a Booster Pack. If you lose, the Ghoul will steal the rarest card in your Trunk. Here are the Ghouls:

Seeker/Rare Hunter/Exodia Duelist
Strings
Odion
Lumis and Umbra
Arkana

To get the following duelists in the game, you have to do certain things:

Mokuba Kaiba: Lose 5 times in a row.

Maximillion Pegasus: Obatin the Toon Wold Magic Card, either by getting it from the Blue-Eyes Toon Dragon pack (which you get when you reach 10,000 points. Check your points by pressing R1 when you're on the city map and looking down the bottom), or using the password, which is 15259703.

Bandit Keith: Beat every normal duelist (they're the ones you don't have to do anything to get, listed at the top) 5 times more than you lose to them.

Marik Ishtar: Once you have defeated all of the Ghouls, you will be taken to the Ghoul's Hideout. There, you will duel Dark Tea, then Seeker, then Strings, then Odion, then Dark Joey.

Finally, you will be led into the final Chamber, where you will duel Dark Marik. If you defeat him, the Good Marik will come out. You will go back out into the City, and continue duelling as normal.

Except you can now duel the Good Marik on the streets, and you can duel all of the Rare Hunters on the streets, without waiting for the Ghoul Appearances. And sometimes when you duel Good Marik, the bad Marik will come out.

Shadi: Defeat everyone at the Ghoul's Hideout.

Duke Devlin: Defeat everyone listed above at least once.

There is also rumoured to be a God Duelist who uses an Egyptian God Card, but that's wrong.

It might be in a different game, but there is no God Duelist, and you can't get Egyptian God Cards in this game. Sorry!