You can use the script below to find how many customers are in each tax group.
select b.taxing_group_code as Tax Group,
b.inactive as "Tax Group Inactive Y/N", count(a.tax_group_id) as "Number of Sites in Tax Group"
from ar_customer_site a, ar_taxing_group b
where a.tax_group_id=b.taxing_group_id
and a.customer_id<>1
group by b.taxing_group_code, b.inactive, a.tax_group_id