Question

what is called a number of trees

Answer 1

A group of trees may be referred to as a forest, grove, orchard, stand or thicket. These names are usually awarded depending on the number of trees being grouped together and the type of trees as well.
Or in easy language A group of trees is called a grove or a cluster of trees or copse of trees.
 
hope it helps you........

Answer 2

a group of trees can be known as a forest. where lots of  wild trees are grown without any human effort .