You can resurrect as many times as you can afford/find Resurrection Powders or Amulets/Ankhs of Life with no damage to stats to the character in question, though he/she is resurrected with only partial hp and stamina. Or you can use the Resurrection spell once your casters are high enough to cast it and then even the cost is eliminated. To the best of my knowledge, there is no maximum number of times you can resurrect any one character but it IS possible that there is one that just hasn't been discovered till now -- I know I had characters that got killed at least 12 times my first time through and I had no trouble resurrecting them.
This is changed from the older Wizardrys where you permanently lost a point of vitality when you resurrected.