Can someone help me to convert the excel formula to use it in Tableau
=sum(aggr(SUM(distinct SALES_TEAM_SIZE),[SALES TEAM]))
In Tableau sums are automatically classified as aggregations.
I am not entirely sure what you data looks like but based off your question I will assume that Sales Team Size is an integer and Sales Team is a varchar or text field.
For simplicity of explanation I will describe this process by making three calculations:
First lets make a calculation to get the distinct count of SALES_TEAM_SIZE and name it [dc_sales_team_size]:
COUNTD([SALES_TEAM_SIZE])
Now we'll make one to get the total number of SALES TEAM and call it [c_sales_team]
COUNT([SALES TEAM])
Create one last calculated field, name it [st_total] and add these two new fields together.
[dc_sales_team_size] + [c_sales_team]
Drag [st_total] to your worksheet.
Read more on Tableau Aggregations here.