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Tesla operates four production and assembly plants, such as: Tesla Factory in Fremont, California (TS); Giga Nevada near Reno, Nevada (GF1); Giga New York in Buffalo, New York(GF2); and Giga Shanghai in Shanghai, China(GF3) opened at the end of 2019. As of 2019, Tesla sells Model S, Model 3 (supposed to be E, but was patented by Ford), and Model X.

Hypothetically, the monthly capacities (in 1,000 of units) of the four plants are as follows. So, for example, if I run GF1 for a month, I produce 8 Model S, 16 Model 3, and 1 Model X. The line is set up like a "flow shop", meaning that production is rigid and I cannot choose to only produce 8 Model S and not make the 16 Model 3 and 1 Model X.

The total demand for Model S is 200,000 units, for Model 3 is 500,000 units, and for Model X is 100,500 units. Moreover, the monthly operating cost for TS, GF1, GF2, and GF3 are $85,000, $60,000, $60,000 and $60,000, respectively. How many months should each plant be operated in order to fill the total demand and keep the operating cost at a minimum?

Q: The number of decision variables are ?

and the number of constraints (not including any non-negative or integer constraints) are ?