Training Unit Schedule

I have a class with a maximum capacity of 28, sometimes I have as little as 3 students, and other times it could be a full class of 28 students. My issue is this, I have 5 training units for the class, and I want to create an excel schedule that would equally and fairly assign a day for each one of my students to use one of the 5 units throughout the month. So if let's say I only have 5 students, then all 5 students will practice daily, but If I had 10 students, then each student will practice every other day, and 15 will be less and less, and so on so forth. What I need is a dynamic schedule that can change based on the number of students in the month. any solutions for this??