Big topic — I love breaking this down. If you want the arcs of 'One Piece' in order, people usually group them into
Sagas (big blocks) and then arcs (smaller story beats inside those blocks). Below I give the commonly used saga-by-saga episode ranges and rough episode counts so you can see how long each chunk runs. Keep in mind a few episodes here and there are fillers or recaps, so numbers can wobble by a couple episodes depending on the source.
East Blue Saga: episodes 1–61 (about 61 episodes) — includes
romance dawn, Orange Town, Syrup Village, Baratie, Arlong Park, and Loguetown.
Alabasta Saga: episodes 62–135 (about 74 episodes) — Whiskey Peak, Little Garden, Drum
Island, and the big 'Alabasta' arc.
Sky Island Saga: episodes 136–206 (about 71 episodes) — covers Jaya and Skypiea.
Water 7 Saga: episodes 207–325 (about 119 episodes) — Water 7, Enies Lobby and Post-Enies Lobby (loads of pivotal stuff here).
Thriller Bark Saga: episodes 326–384 (about 59 episodes).
Summit War Saga: episodes 385–516 (about 132 episodes) — Sabaody,
amazon lily, Impel Down, Marineford, and Post-War.
Fish-Man Island Saga: episodes 517–574 (about 58 episodes).
Dressrosa / New World build-up: episodes roughly 575–746 (this block includes Punk Hazard, Dressrosa and the lead-up; around 170 episodes depending on how you split canon/filler).
Whole Cake Island Saga: roughly 747–877 (about 130 episodes covering the Big Mom territory and aftermath).
Wano Country Saga: starts around the late 800s and runs a long, dense story (Wano is one of the longest single sagas in episodes).
If you want a single-number takeaway: the show is organized into sagas that span dozens to well over a hundred episodes — early arcs can be under 20 episodes while later sagas can stretch over 100. For a precise, episode-by-episode list up to whatever the latest airing is, fan wikis and official episode guides keep exact ranges, but this breakdown will get you oriented. Personally, I still get chills rewatching the Enies Lobby and Marineford stretches — huge payoff arcs.