"You have too many cats."
Translation:Du hast zu viele Katzen.
Here the "-e" at the end is a result of the gramatical case called accusative:
Du hast zu viele Katzen. Du hast viele Katzen. is accusative and plural and indefinite article "viele".
Du hilfst vielen Menschen. is dative and plural and indefinite article "viele".
Let's see what will happen with another type of article:
Du hast die zu vielen Katzen. Du hast die vielen Katzen. ~ here you get "-en" at the end because of accusative and plural and definite article "der/die/das".
Du hilfst den vielen Menschen. is dative and plural and belongs to the definite articles of "der/die/das".
Just adding that "Du hast die zu vielen Katzen" isn't a correct sentence ("You have the too many cats"), but apart from that, the grammar is correct. ("Du hast die vielen Katzen" = "You're the one who owns all those cats" works fine.)
If you substitute "many cats" with "large jackets", it works better: "Du hast die zu großen Jacken." = "You have the too large jackets (I've been looking for)."
(...and accordingly, "Du hast die großen Jacken." = "You have the large jackets.", and "Du hast (zu) große Jacken." = "You have (too) large jackets.")