Does the mass of reactants stay the same during a chemical reaction?

No! Most of the time not. If the mass of the reactants does stay the same, no chemical reaction has taken place.

The mass of the reactants might not go down all the way, but if a reaction takes place even in part, the mass of the reactants becomes less than what it was before the reaction took place.

yes

Explanation:

the reactants are what the chemichal reatcants are using to make what ever it is making