How much master data do we need before we can start?

Less than an ERP implementation requires, and it does not need to be perfect. The essentials are a usable demand history, typically around two years, current stock positions, open and in-transit orders, and the constraints that shape orders: lead times, minimum order quantities, lot sizes and order frequencies. Gaps are normal. During data mapping each missing field is assessed, and many can be held and maintained in the planning tool rather than blocking the project until the source system catches up. A useful side effect is that this exercise produces a precise list of master-data gaps, which is exactly the input your ERP programme will need later anyway.