When is the marginal cost horizontal?
The marginal cost is horizontal when the cost of making the next unit is the same as the cost of making units prior to it. So, if you had a sixth unit in the equation above, and it had a marginal cost of $20, and so did numbers seven, eight, nine, and 10, you'd have a horizontal marginal cost curve because you'd plot each one at $20.
Most of the time, a marginal cost curve is either falling or rising because lots of variables keep costs moving up and down. Those variables include how well you're scaling, the costs of continuing to build more facilities, and the bulk discounts you may start to receive as you scale up.
However, a few business models tend to lend themselves to horizontal marginal cost curves. For instance, a software company or streaming service can reach minimal production costs for their offerings at a certain point. Both achieve horizontal marginal cost curves through massive sales of products with minimal costs when spread across the entire customer base.