Applying that rule of thumb to how many plants per pump formula, multiply the gpm x 20 (minutes per hour) = gph at this pump rate x 24 = gallons per day pumped without over-stressing the pump. Multiply that # 4 for plants up to 3' tall, x2 for plants 6' plus (assuming 1/4 or 1/2 gallon per day per typical plant use).
The above assumes equal watering throughout the day and doesn't take into account fluctuations based on temperature, lights on and off, purging lines, etc. Other variables to consider are temperature, humidity, size of plants, strain, growing style…
As a general rule of thumb:
Our small system : 100-200 plants
Our high flow: 200-500 plants
Our super-high flow: up to 1000 plants