What is the coordination number

[Cr(NH3)2(en)Cl2]Br2
I know it is 6, but how do you get that?

NH3 is monodentate (it donates a single pair of electrons to the Chromium ion). There are two NH3 molecules in the complex, so a total of 2 pairs of electrons coming from NH3. Ethylenediamine (en) donates two pairs of electrons to the Chromium ion (this means it is a bidentate ligand). Chlorine is also monodentate so it donates 1 pair of electrons to the Chromium ion. There are two chlorine molecules, so a total of 2 pairs of electrons coming from NH3. If you add up the number of pairs of electrons donated to the Chromium ion, you get 6.

You might want to memorize the common polydentate ligands. Ethylenediamine (en) is very common. It is bidentate. Other ligands, like EDTA, have four pairs of electrons that they can donate.