Can someone take the MBTI Assessment too many times?
In terms of taking assessments too often, there is no harm in it, but the frequency of taking any given assessment draws the validity of subsequent takings into question–especially if there is feedback before the later times in which someone takes the test/assessment. Myers said always trust your first taking of the MBTI the most, so she knew that this tendency to over-think the items and to answer them from a place of knowing was an inhibiting factor. There is no harm in over-assessing, but psychometrically, it is not desirable. If someone in a class/training of mine knows his/her preferences and can tell me with certainty/clarity, I let him/her not retake the instrument to avoid this over-assessment issue.
If the over-assessment is, rather, referring to someone giving himself too high a score, this is a major issue with trait assessments designed to measure amounts of behavior, development or skill level. To guard against this kind of over-assessment danger, most good trait tools have scales within them designed to make sure that respondents are telling the truth–even if that truth is unflattering. This kind of over-assessment, however, has no bearing on the MBTI, given that type is not about measuring amounts of anything. It is about sorting into this bucket or that one–and the number or degree is very much irrelevant.