We find that the best advice is almost always to fill all usable roof space.
Asking how many solar panels you “need” is a misleading question, because it implies that your solar array will meet/match the usage in the house. You can have an array of panels that are forecast to produce 3,500 kWh, and if your usage is 3,500 kWh then this would seem like job done at first glance. But, unless your usage is highly unusual in a very specific way, the seasonal variation will scupper you. An array that yields 3,500 across the year will yield around 480 kWh in July, but around 90 kWh in December.
So whatever size array you get, it’s highly likely that you’ll produce more than you need in Summer, and less than you need in Winter. So you’ll always need the grid.
That all said and done, once you’ve paid for the inverter, battery storage, scaffolding, project management, labour, insurance etc, the price of the panels is a fairly small part of the equation.
We can install 10 panels, a mid-range inverter and a mid-range 5.76 kWh battery for a total price of £8,340.00. Bump that up to 20 panels and you’ll only see the price go up to £10,240.00. For an extra 23% in cost you’ll have twice as many panels, producing twice as much power. Even if that means a higher proportion of the power that you generate is exported at 15p rather than consumed (saving you ~25p), it’s still almost always the best advice to fill all usable roof space.