There is no cruise line which will have loads of formal nights. There are however, more posh and luxuries cruis elines which are Cunard and Celebrtiy which may fit your criteria better.
The QEII and QMII usually have more formal nights. I went on QEII from Southampton to New York for 6 Nights, and 4 of those were formal. It was fantasti.
Don't know about the other Cunard ships but you can happily book the QE2. As my wife said "It's like going to a wedding almost every day". Whether that is a good thing or not is open to debate.
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