In the One Piece tenkos, Oda would make a volume to volume story summery
He would say for example
"Events from Volume 1 to Volume 38:
Gold D. Roger was excuted, and One Piece was up for grab. Countless of pirates rise their flags and set into the Grand Line.."
So why olny few pirates are after the One Piece? Why not countless? Let's elaborate
Red-Haired Shanks: After the One Piece
Luffy D. Monkey: After the One Piece
Alvida: Just sailing around East Blue untie she meets Buggy
Buggy the Clown: He was after other treasures
Gaimon: Was a cabin boy 20 years ago, we don't know what his former pirate crews are after.
Kuro: Does'nt care
Don Krieg: After the One Piece
Zeff: Had been in the Grand Line for one year but did not go after One Piece
Mihawk: Does not care
A pirate captain who treated to take over Baratie in the Sanji flashbacks in the last Baratie chapters: He may have been going after One Piece
Arlong: Too busy sitting on his ass and dreams about conquering the East Blue, One Piece does not interst him.
Pirates who were threating Tashigi in Louge Town: They plan to go into the Grand Lime and capture One Piece but Smoker caugth their captain and they lost.
Dorry and Borgy: Busy figthing. They don't know One Piece exist beacuse there'er stuck on the same island for 100 years
Wapol: Not a real pirate
Pig Pirates: Nothing much about them, they last olny 2 pages before getting dryed up by Crocodile. They may be after One Piece
Baroque Works: Just bunch of bounty hunters led by a former pirate
Crocodile: Only wants a military nation
Whitebeard: Ace is searching One Piece for him
Bellemy: He does not believe in dreams, he would not want One Piece
Roshio: Killed off by Bellamy so I don't know.
Monbran, Shouchou, and Masira: Olny bunch of savage hunters, they only search treasures under the water, they don't care about One Piece
Blackbeard: He wants to be a Shichibukai so One Piece does not interst him
Donquixote: Wants to start a new pirate age. He does not care.
Frog Fang Pirates: They may have been after One Piece but Foxy stripped them of their pride
Foxy: stuck in his own pride. He does not care about One Piece
Mikazuki: May be after One Piece but we never knew
Pirates in the Hatchi and and Jango mini-stories: Not much known
See what I mean? There's not that many pirates after One Piece.