which cruiseline ? some are more formal than the others

2 formals are enough - gowns, silk pants, even cocktail dresses are all good.

I assume your cruise is with one of the mass market cruiselines - that is, not the up-market lines such as Radisson 7 Seas, Silver Seas, etc.

Even with the mass market lines, some lines are more formal - their passenger mix are more upscale, Holland America, Celebrity, and certain Princess ships.

Royal Caribbean, Carnival, Norwegien are very casual, especially the latter 2.