Use normal Comer when talking about eating in general.
Use Reflexive Comer when talking about eating something specific, or a specific quantity.
Jimmy ate tacos.
Jimmy comió tacos.
Not a specific taco nor a specific quantity, so use normal Comer.
Jimmy ate the taco.
Jimmy se comió el taco.
“The” taco is a specific taco, so use reflexive.
Jimmy ate two tacos.
Jimmy se comió dos tacos.
Specified quantity, so reflexive comer.
Jimmy ate a lot of (many) tacos.
Jimmy se comió muchos tacos.
Specified quantity (many), so reflexive Comer.