Personalized electromagnetic simulation models can be generated by segmenting MR-images. However, it is unclear how many tissue types are required for accurate 7T head models. Here, a clustering approach is used to determine the error in the simulated pTx SAR for models with different numbers of tissue types (clusters). Models consisting of only four different tissue types plus air were found to consistently generate low errors for human body-models of different ages and genders. Using the proposed method, it should be possible to operate scanners closer to the true SAR limits due to improved estimations of the actual patient-specific SAR.