Why "The garden has more trees." isn't correct? In a previous exercise "mehrer" was translated as "more". Why not here too?
I think that would be "Der Garten hat mehr Bäume"
mehr means more something and is not declined
mehrer+ is declined and means a number of (eg several, quite a few)
you would use "mehr" in a comparative sentence. more than/mehr als. or just as is "i have more/ich habe mehr". but when it is declined, it becomes something for the next word, here "Baueme/trees".
Isn't "several" a synonym for "many"?
Not really, with several you say there's more than two, three... (undefined number) but not really a great quantity as you do by saying "many".