。
Look, there are tons of mangoes growing!
Literal
Look mango [subj-が] lots are-bearing [emph-よ].
なる here is the verb 'to bear (fruit),' not the more familiar 'to become' — context (fruit on a tree) disambiguates. ~ている marks the resulting state ('they have grown and are now hanging there'), a classic change-of-state use of ~ている. The opening ほら 'look / hey' is an attention-grabbing interjection used when showing someone something.