How to count the number of rows with specific data in mssql

SELECT 
    COUNT(*) As ExistCount 
FROM 
    dbo.Items
WHERE
    StockExists='T'

So your query should work.

Result:

EXISTCOUNT
    2

Demo

Update

How to perform another such Count operation and add them together in one row, for example, Select count(StockExists) From [Items] where StockExists='T' and Select count(Type) From [Items] where Type='Cellphone' ?

You can use SUM with CASE:

SELECT 
  ExistCount = SUM(CASE WHEN StockExists='T' THEN 1 ELSE 0 END) ,
  CellphoneCount = SUM(CASE WHEN Type='Cellphone' THEN 1 ELSE 0 END) 
FROM 
    dbo.Items

Result:

EXISTCOUNT    CELLPHONECOUNT
    2               2

Demo