This offer ends either when all 15,555 are claimed, or on 18th July, whichever comes sooner. It can't be used with any other offer for the Go On. One per household. Also don't forget it's online only! So if you're doing an online shop anyway, it's well worth it.
Here's a link to the offer: https://goo.gl/5j7nb9
Whilst there, it's also worth looking at sites such as https://www.vouchercodes.co.uk/sainsburys.co.uk and Sainsbury's own website https://goo.gl/zaZa8E for extra offers, particularly if it's your first shop!
Hope this helps!