Number sense studies have indicated that the development of number sense should be the focus of primary school mathematics education. The literature review revealed that learner performance is linked to teacher subject knowledge and that teachers' confidence in doing and teaching mathematics influences the way they teach and their willingness to learn mathematics. This study was motivated by the poor performance of Namibian primary school learners in both national and international standardised assessment tests and explored the number sense of 47 final-year primary school pre-service teachers (PSTs) in Namibia. The data in this mixed method research design were obtained from a number sense questionnaire, a written computations questionnaire, a mental calculations questionnaire and the McAnallen confidence in mathematics and mathematics teaching survey (MCMMTS). Six PSTs, randomly selected from the 47 participants, were interviewed to determine their use of number-sensible strategies. The overall results of this investigation revealed that the final year primary school PSTs demonstrated limited number sense and possessed very few of the indicators of number sense. Unexpectedly, the confidence survey showed that they were confident in their ability to do and to teach mathematics. This study exposed one reason for the low standards of performance of Namibian learners in mathematics and the lack of improvement over the last few decades. It indicates a need for teacher training institutions to identify the mathematics that teachers should know and the ways in which teacher understanding of subject content has to be transformed to enable them to develop the number sense of primary school learners.