By law, A.R.S. 32-2101(54), you are considered a subdivider if you own or have owned 6 or more lots in a single platted subdivision and offer any number of them for sale. All subdividers are required to obtain a Public Report(s) prior to offering lots for sale in accordance with A.R.S 32-2181 et seq. and Commissioner's Rule R4-28-B1207.
Upon acquiring fee title or an equitable interest in the 6th lot within a single platted subdivision, a Public Report is required prior to its sale. You could purchase and sell 2 lots every five years and as soon as you acquire an interest in the 6th lot and offer the lot/lots for sale, you would be in violation if you did not obtain a Public Report. If you hold fee title or an equitable interest in 6 or more lots at one time, a Public Report is required prior to offering any of the lots for sale.