The first cruise is seven nights, so two formal nights. The second cruise is fourteen nights, so four formal nights. Therefore, six formal nights.
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OAlgo, you'll probably find the dress code adhered to on formal nights for the MDRs, Epicurean and the Crow's Nest. You'll be fine in smart clothing in the Glass House, Beach House. Not sure about Sindhu.
Edit:
On a previous Britannia cruise we ate in Sindhu on Valentine's Day, which was a formal night. We were dressed up, but others were wearing shirt and ties, and some had on shirts without ties. That though was in the Caribbean.