You can use the Linux Bash core utility split:

split -b 1M -d  file.txt file

Note that M or MB both are OK but size is different. MB is 1000 * 1000, M is 1024^2

If you want to separate by lines you can use -l parameter.

Update:

a=(`wc -l yourfile`) ; lines=`echo $(($a/12)) | bc -l` ; split -l $lines -d  file.txt file

Another solution as suggested by Kirill, you can do something like the following

split -n l/12 file.txt

Note that is l not one, split -n has a few options, like N, k/N, l/k/N, r/N, r/k/N.

readarray -t lines < file.txt
count=${#lines[@]}

for i in "${!lines[@]}"; do
    index=$(( (i * 12 - 1) / count + 1 ))
    echo "${lines[i]}" >> "file${index}.txt"
done

awk '{
    a[NR] = $0
}
END {
    for (i = 1; i in a; ++i) {
        x = (i * 12 - 1) / NR + 1
        sub(/\..*$/, "", x)
        print a[i] > "file" x ".txt"
    }
}' file.txt