Two, if they are NOT in love and/or despise each other to the core before marriage, then they'll get married for sure. As soon as they fall in love, they will be torn apart by circumstances.
The first rule applied to couples like Neev/Prachi, Prem/Mukti, Sujal/Kashish, Anurag/Prerna, Robbie/Kasturi, and Karan/Nandini.
The second rule applied to couples like Milind/Prachi, Jai/Bani, Bajaj/Prerna and Shiv/Gauri. I'm afraid Ranbir/Kanya from Kuch Is Tara are heading down the same path.
With the exception of Mihir/Tulsi and Kavya/Anjali, I have yet to find a BT couple that fell in love and got married without facing any major hurdles.
Ekta's shows are so predictable. I saw the whole Neev/Prachi/Milind/Ayesha quad coming a mile away. I always knew Ayesha and Prachi's friendship was temporary, since BT heroines don't share a good rapport with their sisters for too long.