Depends entirely on the size of battery you want to use and how long you’re going to want to use the gear between charges. 2 inverters/batteries will obviously double the cost and double the size and weight, so I’d always recommend a single inverter with a battery or two of the size to fit your needs. A 12v 12ah battery, which should last all day on a single charge, can be had for under £20 from places in the UK such as Toolstation. Your biggest current draw will be your amp, just be sure that the inverter is more than big enough to handle your total current draw.