How many party members are there in Dragon Age 2?

So far I have discovered only four: Varric, Bethany, Aveline and Merril. Are there any more?

There are 10: 6 mandatory, 2 optional, and 2 via DLC/Signature Edition.

Mandatory

These companions you get along the way of the main quest line:

Bethany and Carver: acquired at the start of the game

Aveline: acquired during the Prologue

Varric: acquired at the start of Act I

Anders: In Act I, after Tranquility quest

Merrill: In Act I, after Long Way Home quest

Optional

These companions require you doing something extra to secure them:

Isabela: Complete Fools Rush In quest

Fenris: Complete A Bitter Pill quest

DLC/Signature Edition

You get these through DLC (or the Signature Edition, which includes some DLC):

Sebastian: Acquired through The Exiled Prince DLC

Mabari Warhound: Acquired through The Black Emporium DLC. Acts as a fifth party member like Ranger pets did in Dragon Age Origins, not a full-fledged companion.