Cell incremental fill up to a VARIABLE number

in K4
=IF(K3="","",IF(k3+1<=M$2,K2+1,""))

in L4
=IF(ISNUMBER(K3),L$2,"")

and copy down the column

You havent said what happens AFTER K3=M3, ie what happens about the value in M4 M5 M6 etc ?

-ONLY M3 matters. The rest of column M and N are IRRELEVANT.
In this example, for M3=120 I would like the following to happen automatically:
-Column K is filled starting from K3=0 till K123=M3=120 (K4=1, K4=2, K5=3, etc.);
-Column L is filled starting from L3=100000 till L123=L3=100000; (L"y"=L3);

That's all. But I remind you that M3 is a variable value >0 so columns K and L will always be different.

Thank for your reply but I'm sorry to say the commands you typed don't work.

Thank you once again,
F

Revised formulas

in K4
=IF(K3="","",IF(K3+1<=M$3,K3+1,""))

in L4
=IF(LEN(K4)>0,L$3,"")

Comma and semi-colon separators are locale dependent, comma works fine on my formulas, may be different as you're in Germany.

"Done automatically" - Then you need VBA which I'm not an expert on.