The basic number of attempt test is 50. My question is if this number is reseted for different test?
No, it isn't. Test attempt usage doesn't depend on the Google Form being used.
The basic account has 50 attempts, is that the same teacher can have multiple tests with the same 40 students or so?
Yes. It doesn't matter if it is 40 students or 20 students. What matters is the total number of test attempts.