Most methods that take ids in the Stack Exchange API will take up to 100 of them in a single go. This allows applications to batch work and thereby avoid unnecessary round trips, which can be a significant user experience win on slow or high latency devices. Those methods with different vector limits will mention that in their individual documentation.
When passing a vector, sepeate each id with a semicolon. For example, /users/1;2;3;4;5?site=somesite would fetch users with ids 1 through 5 on somesite.
Note that for caching and throttling purposes, vectors are considered unordered. That is,
/users/1;2;3 is semantically identical