Instead of using sample size to identify the number of observations you need, factorial designs are expressed in terms of the number of replicates. Replicates are multiple experimental runs with the same factor settings (levels). For example, if you are doing a full factorial with three factors, one replicate would require eight corner points. The base design would include all combinations of the low and high levels for all factors. Each time you replicate the design eight runs are added; these runs are duplicates of the original eight corner points.