If we are talking about the service work that is to be carried out by a 'competent person' then, currently, there must be at least two services per annum because the BS recommends the period between successive service visits is six months. However, where the system contains vented batteries then it will be only three months, or except where the result of a risk assessment determines shorter service intervals are required, or except where the manufacturer of any particular piece of equipment recommends shorter service periods.
Because not everything on the system needs to be 'serviced' every visit then it means that each visit doesn't necessarily include exactly the same, or the same amount, of service work.
Indeed, some elements only need to be carried out only once annually. But nothing has a service interval of more than 12 months
Therefore each service can consist of different amounts of service work, or the total recommended annual service can be equally spread over the total number of visits required in twelve months.
Therefore the minimum number of visits in twelve months will always be 2 (or 4 with vented batteries)
Simples!
p.s. - I agree with Anthony B that the last BS amendment resulted in the number of necessary visits being reduced down from four visits to only two visits per year (if no vented bat..... etc. etc.) I also do have vague recollections of servicing only once per year some 25+ years ago, but I can't remember if we were doing this to a code of practice or to a customer's requirements.