How many dps healers and tanks are needed to kill the Lich King

On 10man, you generally will want 2 tanks, 3 healers and 5 DPS. On 25man, the suggested number is 3 tanks, 5-6 healers and 16-17 DPS.

These numbers will vary depending on how experienced and geared your players are. With high dps, the adds live for a shorter time, so one tank could tank more of them. When healers are struggling, you may want to bring in an extra healer, and so on.