[Andrew Musau]

Then you are counting firms in a category in at least one sample year. Here is an example with some observation counts modified (highlighted):

* Example generated by -dataex-. For more info, type help dataex
clear
input double(year NPC_FIC pempl)
2010 500000001 3
2010 500000002 5
2011 500000002 4
2012 500000002 5
2013 500000002 4
2014 500000002 1
2015 500000002 2
2016 500000002 2
2017 500000002 2
2018 500000002 1
2010 500000033 2
2011 500000033 2
2012 500000033 2
2013 500000033 1
2014 500000033 1
2015 500000033 2
2016 500000033 1
2017 500000033 1
2018 500000033 1
2019 500000033 1
2010 500000050 2
2011 500000050 1
2012 500000050 1
2013 500000050 1
2014 500000050 2
2015 500000050 2
2016 500000069 1
2017 500000069 1
2019 500000073 2
2010 500000083 1
2011 500000083 1
2012 500000083 1
2013 500000083 1
2014 500000083 1
2015 500000083 1
2016 500000083 1
2017 500000083 1
2018 500000083 2
2019 500000083 2
2015 500000101 1
2016 500000101 1
2017 500000101 1
2018 500000101 1
2019 500000101 1
2010 500000104 2
2011 500000106 1
2011 500000113 1
2012 500000113 1
2013 500000113 1
2014 500000113 1
2010 500000119 8
2011 500000119 9
2012 500000119 8
2010 500000121 2
2011 500000150 1
2012 500000150 1
2013 500000150 1
2014 500000150 1
2010 500000156 2
2016 500000156 1
2017 500000156 1
2018 500000156 1
2019 500000156 1
2010 500000157 4
2011 500000157 3
2012 500000157 2
2010 500000165 4
2011 500000165 4
2012 500000165 3
2013 500000165 3
2014 500000165 3
2015 500000165 3
2016 500000165 3
2017 500000165 3
2010 500000180 1
2010 500000198 3
2011 500000198 3
2012 500000198 2
2013 500000198 2
2014 500000198 2
2015 500000198 2
2016 500000198 2
2017 500000198 20
2010 500000201 1
2011 500000201 1
2012 500000201 1
2013 500000201 1
2010 500000204 2
2011 500000204 2
2012 500000204 2
2013 500000204 3
2014 500000204 3
2015 500000204 3
2016 500000204 23
2017 500000204 3
2018 500000204 1
2019 500000204 2
2010 500000212 1
2011 500000212 1
2012 500000212 299
end

bys NPC_FIC: egen tag1 = max(pempl<=10)  
lab var tag1 "1 to 10 employees"
bys NPC_FIC: egen tag2 = max(inrange(pempl, 11, 250))
lab var tag2 "11 to 250 employees"
bys NPC_FIC: egen tag3 = max(pempl>250)
lab var tag3 "more than 250 employees"
egen tag= tag(NPC_FIC)

*TOTAL NUMBER OF FIRMS
table tag if tag, nototal

*FIRMS WITH LESS THAN 10 EMPLOYEES IN AT LEAST ONE SAMPLE YEAR
table tag1 if tag1 & tag, nototal

*FIRMS WITH 11-250 EMPLOYEES IN AT LEAST ONE SAMPLE YEAR
table tag2 if tag2 & tag, nototal

*FIRMS WITH MORE THAN 250 EMPLOYEES IN AT LEAST ONE SAMPLE YEAR
table tag3 if tag3 & tag, nototal

Res.:

. *TOTAL NUMBER OF FIRMS

. 
. table tag if tag, nototal

-------------------------
             |  Frequency
-------------+-----------
tag(NPC_FIC) |           
  1          |         22
-------------------------

. 
. 
. 
. *FIRMS WITH LESS THAN 10 EMPLOYEES IN AT LEAST ONE SAMPLE YEAR

. 
. table tag1 if tag1 & tag, nototal

------------------------------
                  |  Frequency
------------------+-----------
1 to 10 employees |           
  1               |         22
------------------------------

. 
. 
. 
. *FIRMS WITH 11-250 EMPLOYEES IN AT LEAST ONE SAMPLE YEAR

. 
. table tag2 if tag2 & tag, nototal

--------------------------------
                    |  Frequency
--------------------+-----------
11 to 250 employees |           
  1                 |          2
--------------------------------

. 
. 
. 
. *FIRMS WITH MORE THAN 250 EMPLOYEES IN AT LEAST ONE SAMPLE YEAR

. 
. table tag3 if tag3 & tag, nototal

------------------------------------
                        |  Frequency
------------------------+-----------
more than 250 employees |           
  1                     |          1
------------------------------------

[Paris Rira]

Thank you so much, Prof Andrew. Eventually, it worked

. table tag if tag

----------------------
tag(NPC_F |
IC)       |      Freq.
----------+-----------
        1 |    549,865
----------------------

. table tag1 if tag1 & tag

----------------------
1 to 10   |
employees |      Freq.
----------+-----------
        1 |    512,516
----------------------

. table tag2 if tag2 & tag

----------------------
11 to 250 |
employees |      Freq.
----------+-----------
        1 |     76,487
----------------------

. table tag3 if tag3 & tag

----------------------
more than |
250       |
employees |      Freq.
----------+-----------
        1 |      1,639
----------------------