Learn Spanish (Spain): Tengo tres cartas y tú tienes dos. - I have three cards and you have two.

In Spanish, you can usually drop subject words like yo because the verb ending already tells us who is acting (tengo always means I have). However, tú is included here to create a contrast between the two people: I have three, and you have two.

Why does 'tú' have an accent mark over the 'u' here?

The accent mark is used to distinguish tú (the pronoun meaning you, as used here) from the exact same spelling without an accent: tu (the describing word meaning your, as in tu carta).

Doesn't 'cartas' usually mean 'letters'?

Yes, cartas often means letters (the kind you send in the mail). However, it is also the standard word for playing cards. In a restaurant, la carta also means the menu!

Could I say '...y tú tienes dos cartas' at the end instead?

You could, but it sounds repetitive. Just like in English, if the object (cartas) is already clear from the first half of the sentence, you can just use the number by itself (dos) to mean two of them.