Answer
Yes — a Custom Enumeration's combined Client/DB value definitions are capped at roughly 4,000 characters total, and this includes some internal configuration metadata for the enumeration itself, not just the raw value text you've entered, which is why the error appears once enough rows accumulate rather than at a fixed row count. For a genuinely long list of options, consider using a lookup/reference table with a proper LOV instead of a Custom Enumeration, rather than continuing to add rows past this character-based ceiling.