I just got a quote for our community farm from NFU and it was well worth having thought through most of the issues beforehand as they do have a lot of questions to ask.
We were mainly interested in the employers liability and public liability insurances which I assume you'd need too for running a livery. These tend to increase with numbers of employees/number of folk visiting the yard which may be very different to number of horses.
I guess if you're insuring the horses themselves then costs will go up more in line with the number (and value) of horses you have. We ended up just taking out a little bit of 'thirdparty-fire-and-theft' for our breeding sows, because it was so cheap to do but not worrying about insuring the poultry etc.