How do we say: The McDonald's products are good, but when there are more than one McDonald's? Where do we put the " ' " ?
McDonald's is the name of a company, and cannot be altered for grammatical purposes. Therefore, the sentence must be altered to accommodate the proper noun. If you feel that you need to construct a sentence with a plural McDonald's, try one of these:
The products at McDonald's restaurants are good. (my choice)
McDonald's restaurants' products are good. (more awkward, but grammatical)