These two words have completely different meaning but they have the same pronunciation.
mais - more, further mas - but, however
Unless someone is emphasizing its pronunciation and speaking very slowly:
Mas - Mais
There is no difference:
Eu gosto mais de você.
Eu quero comer sorvete, mas estou de dieta.
Oh, and the word 'pero' doesn't exist in Portuguese, that's Spanish 🙋♂️