There's plenty of marinade for 7 or 8 chicken breasts, no problem. I normally put 1kg of chicken in and there's plenty of room for more.
Just as an aside, I now use mickdabass' chicken tikka recipe, and that marinade copes easily with 1kg too.
I use mint jelly myself, bit it's down to personal preference. The sauce can be a bit overpowering if you''re not careful, while the mint is much less so and so more can be added if necessary. It's a bit like the old adage about sugar and salt - you can put it in but you can't take it out again.