Depending on your Audi Allroad's generation, factory wheels and registration, Tyres.co.uk has suggestions for your Allroad’s tyres. The online size guide is a general guide rather than a definitive fitment guide. Verify your tyre pressure placard and confirm approved fitment for the size, load index, speed rating and if it's an XL size, before ordering.
The placard on the driver side B-pillar is the location of tyre size information for your Audi Allroad. The placard contains suggested sizes and load and pressure for the factory recommended wheels for each of the wheel choices; if your wheel choice is not factory then you should check the tyres against the placard and the side wall.
All Audi Allroad models are Quattro, so all four tyres must be of the same size, pattern and construction, with the rolling circumference matched across all four. You do not have to replace all four, but if replacing two would give a huge disparity in tread depth then all four must be replaced. If two are replaced, then they should be placed on the rear axle.