1 , aaa, bbb, ...

1 , ccc, ddd, ...

2 , aaa, bbb, ...

..

10 , aaa, bbb, ...

..

1000 , aaa, bbb, ...

1 , aaa, bbb, ...

1 , ccc, ddd, ...

2 , aaa, bbb, ...

..

10 , aaa, bbb, ...

11 , ccc, ddd, ...

12 , aaa, bbb, ...

12 , aaa, bbb, ...

..

20 , aaa, bbb, ...

awk -F ',' '!seen[$1]++{f=$1".csv"; print h > f};{f=$1".csv"; print >> f; close(f)}' file.csv