Batch. Parameters inside strings?

So this is the first question i am asking here so go easy on me.

set /p num= 
for /l %%a in (1,1,%num%) do set /p line%%a= 
for /l %%b in (1,1,%num%) do (echo %line%%b%)>>file.txt 

The problem is that i try to have a parameter inside a string, so what other way is it to do this?