how to get monthly date in teradata
Teradata supports proprietary syntax for time series expansion:
select begin(pd), column1, column2
from OTH_MBR 
expand on period (from_dt, thru_dt) as pd
by anchor period month_begin
-- if you want to restrict the range of months
for period(ADD_MONTHS(current_date,- 36), current_date )

You can wrap it in a Derived Table/CTE for further processing:
with cte as 
 (
   select begin(pd) as mon, column1, column2
   from OTH_MBR 
   expand on period (from_dt, thru_dt) as pd
   by anchor period month_begin
   for period(ADD_MONTHS(current_date,- 36), current_date )
 )
select mon, count(*)
from cte
group by 1;