Windows 3.1 uses a segmented memory model. Segments are 64K in size. A list needs to fit into a single segment. Therefore, a list on Windows needs to be 64K or less. In practice this works out to about 10K elements for a linear list, roughly half for a property list.
Precise numbers vary with the size of the entries.
In this single list of reserved words are also held all symbols and all parameters passed to global handlers. This list, like others, cannot exceed 64K in size on PCs. A handler stored in a private sprite or cast script is in its own, separate list. This special list persists for the life of the application. If you branch to a new movie prior entries will persist.