It has been suggested that the large cognitive demands during the observation of assessment center participants can impair the quality of the assessors’ ratings. An aspect that is especially relevant in this regard is the number of candidates that assessors have to observe simultaneously during group discussions, which are one of the most commonly used assessment center exercises. The present research evaluated potential impairments of the quality of the assessors’ ratings (construct- and criterion-related validity and rating accuracy) related to the number of to-be-observed candidates. Study 1 (N = 1046) was a quasi-experimental field study and Study 2 (N = 71) was an experimental laboratory study. Both studies found significant impairments of assessors’ rating quality when a larger in comparison to a lower number of candidates had to be observed simultaneously. These results suggest that assessors should not have to observe too many candidates at the same time during assessment center group discussions.