This study analysed how preschool teachers differently enacted the same mathematical activity for preschool children to discern numbers, and how this affected the children's learning opportunities during the activity. The analysis was based on variation theory and Chi's taxonomy of learning activities. Two Swedish preschool teachers' enactment of the same mathematical activity for 27 children aged 4-6 years was studied. Video recordings of what the children were offered to discern were used in the analysis. The results indicate that variations in how the teachers chose to enact the activity produced two different learning opportunities for the children. Differences in what aspects were made discernible were closely linked to the characteristics of the activity implemented. The enactments differed even if the same game was chosen and the same amount of time was used in the play-based activity. In one preschool group, there were few opportunities to discern more than the nominal form of numbers; the other preschool group had an activity focused on all number forms simultaneously. In addition, in the latter group, the children had the opportunity to develop equinumerosity. The results suggest that the activity with limited variation was more appropriate for learning with undeveloped knowledge; the children with more developed understanding required a more varied design. This study contributes to the knowledge of how the design of an activity affects children's learning differently, which is important when planning learning-based preschool activities.