How to find total number of reserved instances under particular tag?
There isn't a straight way. More appropriately, I would say there are couple of things which aren't fully right in your assumptions. 

The EC2 instances aren't directly tied to the Reserved Instances. It is more of a non-technical - pure billing concept and end of the month, AWS counts the number of instance hours and checks it with number instance reservation hours and discounts the billing. This way no instance reservation is linked or associated with the EC2 instances which are running.
Reserved Instances aren't tagging enabled. Only the EC2 instances have tagging support.

To answer your question on the approach, the following the pseudo code would help

Get the list of Reserved Instances Listings (instance-platform, size, availability zone)
Get the list of EC2 Instance and filter it by Tags [group or name]
With the relieved 2 lists [ list of reservations & list of EC2 instance ] - for each of the (instance-platform, size, availability-zone) matched records may fetch you the list of Reservations & associated EC2 instance.
I am query AWS using boto ec2 in python. Firstly I find all reserved instances by get_all_reserved_instances then I am also able to find total count of each instance_type by instance_count. I am trying to calculate total number of reserved instances under tags.
Eg. We have two tags group and name. Then I want to show total number of reserved instances of particular type (Eg. i2.xlarge) under group tag. How to do this, I did not find this in AWS console also ?