A lot depends on whether you are talking about a touch game in training or competitively. If it's training then you can make your own rules up. I personally play these rules -
* Two-hand touch.
* Ball carrier must go to floor, roll once and place the ball back.
* One player from each side must bridge over the ball (ruck).
* Defence must identify pilar/post defenders.
* Infinite tackles.
* Game restarted with a free-kick.

You can add rules as you go along. I sometimes say that the two guys bridging can jackle for the ball. Also, that the attacking player who goes into bridge can pick the ball up if it's "go-forward" ball.

I can't really comment on the competitive rules as I've never played it but would imagine that it's something like 5-tackles then turnover and that it's a turnover if the scrum-half is touched/tackled.

Hope this helps.