With 13 cards on each suit, democratically you need to have at least 7 cards to claim “majority” on the suit (4-3, 5-2). But what consider “good fit” will be at least 8 cards fit (4-4, 5-3, 6-2). While of course the more card is the better when declaring a suit contract, remember that , sometimes, having a double suits fit, playing 4-4 fit as trump gives you more trick than even 5-4 or 5-3 fit as the 5-3/5-4 suit can be used for discarding losers (while 4-4 become trump). Then the important question. There are 4 important types of support, does your bidding system covers it ?
Theoretically, you will play a suit contract 4 times more often than NT contract. Playing a suit contract is a little bit more complicated as the highest card in a suit will not always win if a trump suit is being played. “Trumping” or “Ruffing” become part of the play.
Once everybody has 13 cards on their hand, then the first battle is to win a contract. Just like the real world contract, to achieve the highest contract we will need to do an auction. And may the highest bidder win.
The contract is simple: the partnership whoever win a contract promise to win certain trick with certain suit as their trump card – or they can choose to use without any trump card.
The other partnership/the opponent will try to defeat the contract by winning as much trick as they can so that the other partnership cannot deliver their contract.