A Unix sed needs a line end after a lable and after { } braces. And a line break or semicolon before a }.
A multi-line sed script can be

Code:

sed 'line1
line2
line3'

or packed into one line

Code:

sed -e 'line1' -e 'line2' -e 'line3'

A packed variant for Unix sed looks like this

Code:

n=10 ; echo "ABCD" | sed -e ':lbl' -e '/^ \{'$n'\}/b' -e 's/^/ /' -e 'b lbl'

The b command (branch without a lable) goes to the next cycle, like the next command in awk. The n command is similar but does not increase the line number, as you can see with the = command and some more input lines:

Code:

n=10 ; </etc/group sed -e '=;:lbl' -e '/^ \{'$n'\}/b' -e 's/^/ /' -e 'b lbl'

Code:

n=10 ; </etc/group sed -e '=;:lbl' -e '/^ \{'$n'\}/n' -e 's/^/ /' -e 'b lbl'

Last but not least, this does not always insert 10 spaces. Instead it ensures there are 10 spaces; if necessary it inserts some.