"He has a lot of coats."
Translation:Er hat viele Mäntel.
Because you always mean multiple coats when using viele and same applies to viel. Viele is in pluralis and gets therefore the ending -e.
Vielen is not probably used in German, because it refers to a singular masculine object. There can never be many of one object, the reason why vielen is not used.
ex. "There are many man" would not make much sense, whereas "there are many men" would.
"You used the singular "Mantel" here, instead of the plural "Mäntel"."
C'mon, seriously? I understand the importance of umlauts and that I can substitute them with ae, oe, ue, but I think this is unnecessary. Normally it is marked correct with "Pay attention to the Umlaut", why not in this case? I always think it with umlaut, and when I have access to umlauts use them.
Also, curious if "ihm gehoert viele Maentel" would be correct, and understood the same way to a native German speaker.