What do you think about the 2 mandatory pit stops in this year's Monaco Grand Prix?

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What’s the rational. How many did teams usually make? 1?

Normally they have to make at least 1. Or more precise, they are required to use at least 2 of the 3 tyre compounds.
Monaco is a track where, especially with the current cars, it's next to impossible to overake a car on track. So, strategy (when to change the tyres) plays an important role in making the race more exciting. That's the rationale of this directive that is unique for this particular race.

The amount of pit stops generally depends on the degradation of the tyres. So 1 or 2 mostly.