How many kva I need

Ask for 20kVA and see what they say. Normally its 15kVA per house but you have an HMO.

Most properties have a 80amp main fuse and supplied at 230v so this would give you 18,400kva. Using diversity this should be enough.

This would depend on whether the installation is single phase or three phase and what the incoming supply cable is made of and what its cross-sectional area is. Capacity for an individual domestic property is typically 18kVA. This would be ample for a five-bedroom house with one electric shower and no significant or disturbing loads. It would be worthwhile speaking with a qualified electrician to complete the necessary calculations prior to any application with your Distribution Network Operator (DNO). The DNO will require accurate calculations of your project’s overall load requirement so that they can then design a supply that matches your needs.

With 9 electric showers and electric central heating you have a very substantial potential load. I would suggest asking your electrician to provide a recommendation. Failing that look at maximum demand for your heating. then add power for 3 or 4 electric showers and maybe 5kva for each kitchen. My guess is that you will come out at something like 60 or 70 KVA. I would ask for at least 69KVA, which is 3 phase 100 amps. Also if you have off street parking worth thinking about providing some ev chargers. There are government grants available for this, an ev charger takes 7.4kva max, but you can use load management so that they are only drawing current when other loads are reduced. At least put the infrastructure in now so that it's a n easy upgrade in the future. But I am making lots of assumptions and the actual request should be based on calculations from the actual electrical design.