I am having trouble in finding how many mergers will the subhalo go through after a specific sanapshot. How to find that.
You should be able to modify the numMergers() example function for this purpose.
To load the "future tree" you would typically specify MDB=True (main descendant branch) option to loadTree(), however to count mergers you need the full tree, not just a main branch. So you would perhaps want to modify this and add a new option to load the full future tree.
On the other hand, the simplest approach would likely be to take your subhalo of interest at redshift z, follow it down to z=0, then use the numMergers() function as is, and then discard any which occur before redshift z.