Use the right coolant, keep the system full and bled, and a Miata holds temperature for decades. The type differs by generation; the capacity is modest.
Type: - NA/NB (1990-2005) - a conventional ethylene-glycol coolant is correct; these cars predate long-life formulas. A quality green coolant mixed 50/50 with distilled water is the safe, standard choice. - NC/ND (2006-present) - use Mazda's long-life coolant (FL22) or a genuine equivalent. It is a long-service formula; do not mix it with old green coolant. - Mixing rule - do not blend incompatible chemistries. When in doubt, flush fully and refill with one type.
Mixture and water: - 50/50 with distilled water - gives freeze/boil protection and corrosion inhibition. Distilled only; tap water leaves deposits. - Track cars - some run a higher water ratio with a water-wetter additive for better heat transfer, accepting reduced freeze protection.
Capacity and service: - System capacity is modest - roughly a gallon and a half of total coolant on these four-cylinder cars; buy a gallon of concentrate and mix. - Bleeding is mandatory - the Miata traps air high in the engine. Fill slowly, run with the heater on full, and burp until the level stops dropping, or it will read as overheating. - Interval - refresh conventional coolant every couple of years; long-life FL22 goes much longer but is not lifetime.
Color is a guide, not a spec - match the chemistry your generation calls for and keep it clean.