How to get rows count of internal table in abap?

You can use the following function:

 DESCRIBE TABLE <itab-Name> LINES <variable>

After the call, variable contains the number of rows of the internal table .

There is also a built-in function for this task:

variable = lines( itab_name ).

Just like the "pure" ABAP syntax described by IronGoofy, the function "lines( )" writes the number of lines of table itab_name into the variable.