As a publisher (or self-publisher if you prefer - both terms are the same to the ISBN authorities) you can purchase ISBNs in blocks of a minimum of ten. At least that's the normal situation. However, Bowker in the USA at least and perhaps other ISBN authorities in other countries, may sell you a single ISBN. But beware, because you will pay way, way more that one tenth the cost of a block of ten. For one ISBN you'll probably pay more like the cost of five ISBNs when purchased in a block of ten.
Another way to obtain a single ISBN is to get your book published through one of many companies that sell themselves to authors as a self-publishing service. These organisations exist to help authors, for a fee, to publish their work. Some companies may offer to take the whole self-publishing process from you, while others may offer to do whatever parts you wish to outsource to them. Dealing with the ISBN will be part of this service. But, as with getting a single ISBN from your ISBN authority, it will not be as relatively inexpensive as getting a block of ten from your ISBN agency yourself. You will have to weigh the cost along with the other services that you may be offered as part of a package of services. Another consideration with going along this path is that although these provders style themselves for self-publishers, the fact is that when you use such an organisation, you are simply not being a self-publisher - they will be the publisher as far as the ISBN authority are concerned, as it is they that are the holders of the ISBN that will be designated to your work. If, as a self-publisher, it is important to you that you are the publisher of record, then this route may not be the one for you.
When everything is considered, it may be false economy to try to obtain a single ISBN rather than purchase a block of ten. When you take into account all of the expenses involved in the writing, production, sales and distribution of your work, then the cost a block of ten ISBN numbers may be a very small part of the overall outlay. You may find that you can easily use more of that block of ten ISBNs when you need to produce another edition of your book, perhaps in a different binding, or geared to another market. Perhaps your success will spur you on to produce your next work, or maybe you could collaborate with another author to publish their book too. When you get into the world of publishing, you may find that you enjoy it, and even a block of 100 ISBNs may not seem such a foolish idea!
If you are self publishing, then you may need more that one ISBN - you may need one for the printed version of a book and another for the same book published as an ebook. For ISBN consideration for ebooks see the E-books And ISBN Numbers page.
To learn more about getting ISBNs from ISBN authorities, see the How To Get An ISBN Number page.