$ cat fsplit.sh

#!/bin/sh

if [ "$#" -lt 2 ] || [ ! -f "$1" ]
then
        echo "usage:  $0 inputfile numfiles" >&2
        exit 1
fi

awk -v NFILES="$2" -v FNAME="file_%d.dat" '
        # Do not print the first file -- just count lines
        NR==FNR { next }

        # First line of the second read through the file.
        FNR==1 {        HEADER=$0
                        MAXLINES=sprintf("%d", (NR-1)/NFILES);
                        LINES=MAXLINES
                        next    }

        # skip to the next file and print header if exceeded maxlines
        (LINES >= MAXLINES) {
                        LINES=0;        FILE++;
                        print HEADER > sprintf(FNAME,FILE);     }

        # Print all lines into the current file
        { print > sprintf(FNAME, FILE); LINES++ }

# Yes, we give awk the same file twice.  On the first read, it just counts
# lines.  On the second, it decides which lines go into what file.
' "$1" "$1"

$ cat data
BASENAME STREETTYPE PREFIX SUFFIX HOUSENUMBER
1 jhj jgu gv 36
2 dut jhg hg 54
3 gkl jkl hv 67
4 fjh gfh hg 45
5 hgl hgk hg 73
6 hkj hg yg 79
1 jhj jgu gv 36
2 dut jhg hg 54
3 gkl jkl hv 67
4 fjh gfh hg 45
5 hgl hgk hg 73
6 hkj hg yg 79
1 jhj jgu gv 36
2 dut jhg hg 54
3 gkl jkl hv 67
4 fjh gfh hg 45
5 hgl hgk hg 73
6 hkj hg yg 79
5 hgl hgk hg 73

$ tail *.dat
==> file_1.dat <==
BASENAME STREETTYPE PREFIX SUFFIX HOUSENUMBER
1 jhj jgu gv 36
2 dut jhg hg 54

==> file_10.dat <==
BASENAME STREETTYPE PREFIX SUFFIX HOUSENUMBER
5 hgl hgk hg 73

==> file_2.dat <==
BASENAME STREETTYPE PREFIX SUFFIX HOUSENUMBER
3 gkl jkl hv 67
4 fjh gfh hg 45

==> file_3.dat <==
BASENAME STREETTYPE PREFIX SUFFIX HOUSENUMBER
5 hgl hgk hg 73
6 hkj hg yg 79

==> file_4.dat <==
BASENAME STREETTYPE PREFIX SUFFIX HOUSENUMBER
1 jhj jgu gv 36
2 dut jhg hg 54

==> file_5.dat <==
BASENAME STREETTYPE PREFIX SUFFIX HOUSENUMBER
3 gkl jkl hv 67
4 fjh gfh hg 45

==> file_6.dat <==
BASENAME STREETTYPE PREFIX SUFFIX HOUSENUMBER
5 hgl hgk hg 73
6 hkj hg yg 79

==> file_7.dat <==
BASENAME STREETTYPE PREFIX SUFFIX HOUSENUMBER
1 jhj jgu gv 36
2 dut jhg hg 54

==> file_8.dat <==
BASENAME STREETTYPE PREFIX SUFFIX HOUSENUMBER
3 gkl jkl hv 67
4 fjh gfh hg 45

==> file_9.dat <==
BASENAME STREETTYPE PREFIX SUFFIX HOUSENUMBER
5 hgl hgk hg 73
6 hkj hg yg 79

$