Enough to keep the enterprise operating and compliant through its early period, based on the actual renewal dates and payment schedules in your policies and licences. Figures such as six or twelve months of operating costs are planning examples, not legal requirements. Money held loosely in reserve may also be viewed differently from capital already committed.
Build the reserve from the renewal calendar
Write down when each policy renews, when each licence expires, and when instalments fall due, then fund those dates. That produces a defensible number instead of a round figure chosen for comfort. Ask how the reserve is held and whether it is dedicated to the business, since the treatment of uncommitted funds differs from spending already at risk. Confirm current government charges at the time of each step rather than budgeting from an old figure.
For a share purchase the relevant cost is the price for the business as a whole, established by the seller's financial statements, the equipment schedule and the negotiated agreement, plus any working capital the buyer must add. Purchase price, equipment, inventory, leasehold improvements, professional fees paid to complete the transaction and initial working capital can all form part of the investment where they are genuinely spent or irrevocably committed.