Modpack server requirements
Find a CurseForge pack and see the four facts that decide whether hosting it takes an afternoon or a weekend: Minecraft version, loader, the Java those two require, and whether the author provides a server pack.
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Where the RAM figure comes from
CurseForge has no memory requirement for modpacks — not on the pack, not on its files, nowhere in the API. Any site quoting an exact "recommended RAM" for a pack either copied it from the author's description or invented it. Ours is our own estimate, based on the pack's installed size and the same model the RAM calculator uses, and it is marked as an estimate everywhere it appears. Use it to get started, then check what the server really uses.
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The server pack matters more than the size
A server pack is a separate download from the author with only the files a server needs. If there is one, hosting is unzip and start. If there is not, you build the server side yourself — and client-only mods are not quietly skipped on a server; they crash it at startup one after another, and you track them down by elimination. A pack without a server pack is a noticeably bigger job, however big or small the pack is.
The pack does not choose its Java
The Minecraft version decides it, which is why we can give an exact answer instead of an estimate. A 1.20.1 pack needs Java 17 no matter what the author says; a 1.21 pack needs Java 21. Getting it wrong is the top reason a correctly built pack will not start — and because the error quotes a class file version rather than a Java version, people usually search in the wrong place.