How to increment line counter for line beginning replacements by AWK/...?

I first thought about SED (sed "s/^/COUNTER \&/" /tmp/1.tex) but it is designed for a single line, and I cannot increment the counter itself by sed so thinking now awk because I have great experiences with gawk in integrated approaches. Data What & South Dragon & North Dragon & 5 \\ \hline What & South Dragon & North Dragon & 5 \\ \hline What & South Dragon & North Dragon & 5 \\ \hline Expected output 1 & What & South Dragon & North Dragon & 5 \\ \hline 2 & What & South Dragon & North Dragon & 5 \\ \hline 3 & What & South Dragon & North Dragon & 5 \\ \hline OS: Debian 8.5