Short answer — globally the biggest slot libraries are at specialist international sites such as Videoslots, SlotsMillion and a few large UK/EU casino brands (Dream Vegas, Casumo, etc.). If you’re in the United States, state-regulated operators (DraftKings, BetMGM, Caesars, FanDuel, BetRivers, etc.) usually offer far fewer titles because of licensing — typically a few hundred to a few thousand slots depending on the state. (en.Wikipedia.org)

More detail and what it means for you

• Videoslots — one of the world’s largest slot lobbies; public reporting and industry write‑ups put its library in the thousands (reports have cited ~9,000+ and other sources list 10,000+ titles). This is an international (Malta/Europe) casino, not generally available as a real‑money regulated brand across the entire U.S. market. (recentslotreleases.com)
• SlotsMillion — another “big catalog” operator that highlights large, multi‑thousand game collections (often touted as 1,500+ titles and many providers). It’s focused on international markets and has an especially large variety because it aggregates many developers. (CasinoSlotsMillion.com)
• Dream Vegas / other large UK/EU brands — major UK/EU casinos commonly list 2,000–3,500 total games and thousands of slots because of broad provider lineups. Again, availability depends on your jurisdiction and local licensing. (thesun.co.uk)
• U.S. state‑regulated casinos — major U.S. operators (DraftKings, BetMGM, FanDuel, BetRivers, Caesars) typically offer hundreds to a couple thousand slots per state site depending on partnerships and state rules; DraftKings has been reported with ~2,200+ slots in some state lobbies. If you’re physically in the U.S., these regulated sites are the ones you’ll generally be able to use. (scams.info)

Quick tips for choosing:

• If your priority is sheer quantity and a huge variety of developers, look at international specialist sites (Videoslots, SlotsMillion) — but check whether they legally accept players from your U.S. state. (en.Wikipedia.org)
• If you must play legally in the U.S., pick one of the major regulated operators in your state (DraftKings, BetMGM, FanDuel, Caesars, BetRivers). They offer fewer titles than the biggest global sites but are licensed and support regulated deposits/withdrawals. (scams.info)
• Game counts change often (new releases, licensing deals, geo‑restrictions). If you want the most up‑to‑date ranked list for your specific U.S. state, tell me the state and I’ll look up current game counts and availability. (I can fetch live counts and links.)