I've been using one for about a year, I've never had the need to use all 8 tuners at one but never had any issue using as many of them as I needed. Your line provider might have issues with you using so many tuners as most count 2 tuners as being one box.
I was under the impression the line is per box/ip so it doesn't matter how many tuners you have on your box? I tested a Zgemma H4 triple tuner for a while with my line and never encountered any problems, but as the box was low spec I gave it to my dad LOL
it's down to the provider, you are right that one box is classed as one connection but most boxes have just two tuners, if you're using all your tuners a lot then they may consider it as abusing the service, if it only happens now and again or briefly then they would probably let it go, some may ask you to get a multi box line for so many tuners, basically you need to ask your provider.