Find all occurrence of value from column b in column a
Test Data:  

CREATE TABLE dbo.Table1( columnA nvarchar(255), columnB nvarchar(255),GIDNumber int, Description nvarchar(255));
CREATE TABLE dbo.Table2( columnA nvarchar(255), columnB nvarchar(255),TwrNum int,Description nvarchar(255));

INSERT INTO dbo.Table1(columnA,ColumnB,GIDNumber,Description)
VALUES(5900920132278,5900021321,1,'F-A1120010185')

,(5900920132278,5900021322,1,'F-A1120010165')
,(5900920132278,5900021323,1,'F-A1120010165')
,(5900920132278,5900021324,1,'F-A1120010165');


INSERT INTO dbo.Table2(columnA,ColumnB,TwrNum,Description)
VALUES(2000061232,5900920132278,1,'F-A1120010185');


Original query

SELECT t2.columnA , t1.columnB, t1.description
FROM Table1 as t1
LEFT JOIN Table2 as t2 on t1.GIDNumber=t2.TwrNum;


Result:

columnA columnB description
2000061232  5900021321  F-A1120010185
2000061232  5900021322  F-A1120010165
2000061232  5900021323  F-A1120010165
2000061232  5900021324  F-A1120010165


Adding COUNT() and GROUP BY

SELECT t2.columnB,t2.columnA , count(t1.columnB) as amount_of_matches
FROM Table2 as t2
LEFT JOIN Table1 as t1 on t1.columnA =t2.ColumnB 
GROUP BY t2.columnB,t2.columnA;


Result:

columnB columnA amount_of_matches
5900920132278   2000061232  4




If you also need to check if GIDNumber = TwrNum 

SELECT t2.columnB,t2.columnA , count(t1.columnB) as amount_of_matches
FROM Table2 as t2
LEFT JOIN Table1 as t1 on t1.columnA =t2.ColumnB and t1.GIDNumber=t2.TwrNum 
GROUP BY t2.columnB,t2.columnA;


As a result, if column B matches column A but the t1.GIDNumber=t2.TwrNum  does not match, it will show as no match:

columnB columnA amount_of_matches
5900920132278   2000061232  4




Extra tests:

INSERT INTO dbo.Table2(columnA,ColumnB,TwrNum,Description)
VALUES(2000061232,5900920132279,2,'F-A1120010185');


SELECT t2.columnB,t2.columnA , count(t1.columnB) as amount_of_matches
FROM Table2 as t2
LEFT JOIN Table1 as t1 on t1.columnA =t2.ColumnB 
GROUP BY t2.columnB,t2.columnA;

columnB columnA amount_of_matches
5900920132278   2000061232  4
5900920132279   2000061232  0

INSERT INTO dbo.Table1(columnA,ColumnB,GIDNumber,Description)
VALUES(5900920132278,5900021321,1,'F-A1120010185');

SELECT t2.columnB,t2.columnA , count(t1.columnB) as amount_of_matches
FROM Table2 as t2
LEFT JOIN Table1 as t1 on t1.columnA =t2.ColumnB 
GROUP BY t2.columnB,t2.columnA;

columnB columnA amount_of_matches
5900920132278   2000061232  4
5900920132279   2000061232  1




Adding the description column:

SELECT t2.columnB,t2.columnA , count(t1.columnB) as amount_of_matches, t2.Description
FROM Table2 as t2
LEFT JOIN Table1 as t1 on t1.columnA =t2.ColumnB   and t1.GIDNumber=t2.TwrNum 
GROUP BY t2.columnB,t2.columnA, t2.Description;


Result:

columnB columnA amount_of_matches   Description
5900920132278   2000061232  4   F-A1120010185
5900920132279   2000061232  0   F-A1120010185