It seems you can’t buy just one ISBN number. You can buy ten for £118.68 or a hundred for £256.32. That’s either £11.87 each or £2.56 each. They are not available in multiples of twenty or fifty. Decisions ….
I will need two for ‘The Rise of Serge and the Fall of Leo’ because I intend to publish it as a paperback and as an e-book, and you need a different ISBN number for each format. Then if I decide to resurrect and modernise some of my past novels (replacing phone kiosks with mobiles, getting rid of cars they no longer make and cameras that use films that need to be developed) I will use up all the ten and will need another ten for the novels I write in the future. That will make the cost of twenty £237.36 when I could have a hundred for an extra £18.96. I think that should be plenty! But £256.32 does seem a lot to spend on, well, ISBN numbers.