How many formal nights on a 7-night Caribbean? What to actually pack

Getting ready for my first 7-night Caribbean sailing and totally confused about formal nights. The cruise line website says "2 formal nights" but different people online are telling me different things about what that actually means in practice. From what I can gather after a lot of research: Most 7-night Caribbean sailings on the major lines (Royal Caribbean, Carnival, Norwegian, Princess) have 2 formal or "dress to impress" evenings. These are typically on nights 2 and 5 on a standard Western…