Number of Nodes in Isolation Forest
(ii) means after splitting the data you end up with a single instance in the node, so there is nothing left to do

(iii) means after splitting the data all the samples in the node are identical ("duplicates"), and again there is nothing left to do


  Assuming all instances are distinct, each instance is isolated to an
  external node when an iTree is fully grown, in which case the number
  of external nodes is n and the number of internal nodes is n − 1; the
  total number of nodes of an iTrees is 2n − 1


Isolating an instance means splitting the data until that instance ends up in a leaf by itself. If you have $n$ instances, you will need $n$ external ("end") nodes. One for each instance. The internal nodes are all the nodes before the end nodes, including the root that contains all the instances. One can show that their number is $n-1$. For example, if you have $n=4$ you can split it in (4)-(2,2)-((1,1)(1,1)). So you have $1+1+1+1=4=n$ end nodes, and $1+2=3=n-1$ inner nodes. 

Of course, the splits don't have to be equal. It could also be something like (4)-(3,1) Where you then split the (3) into (2,1) and the (2) into (1,1) But the counts will be the same. You'll have 3 inner nodes (4),(3) and (2) and 4 external nodes. (Best to draw it out)

The total is therefore the number of end nodes plus the number of inner nodes $n +(n-1) = 2n-1$.