13 Card Rummy is the format every major Indian rummy platform runs by default. Two standard decks plus two printed jokers are shuffled together. Two to six players each receive 13 cards. The objective is the same as the rest of the variants: form valid melds and declare. The session length is short, the rules are simple, and the strategy is bounded.
Why 13 Card is the default
It is the easiest format to teach, the fastest format to play, and the most accessible format on every platform. Every other variant is a modification of 13 Card. If you have never played rummy online before, this is where you start.
What 13 Card teaches you
The format rewards three skills: reading the discard pile, holding the right cards, and knowing when to drop. Because the deck is small and the sessions are short, the sample size for skill development is also small. You can play twenty 13 Card hands in an hour. That is twenty opportunities to read the geometry of a hand and adjust your next decision.
When to graduate from 13 Card
When 13 Card stops surprising you, you are ready for either 21 Card (a longer-form variant with more meld categories) or Pool Rummy (a multi-hand format with elimination). Both require the same mechanics as 13 Card plus a more patient temperament.