The number of straps you need depends a lot on your carrier's requirements and the kind of freight you haul and the kind of truck you drive. If you are in a fleet owner's truck, it will likely come pre-equipped with a number of them.
If you tell us what kind of truck you'll drive, your carrier, and your freight-type expectations, folks here can give you a better answer.
F.Y.I., most manufacturers color code their ratchet straps in three lengths, yellow for 12 feet, gray for 16 feet, and blue for 20 feet. Custom made straps are also available.
Another strap feature to note is the length of the short or fixed end. Most are 2 feet or more at the fixed end. That works for big boxes and skids. For small items, a shorter fixed end is desirable. Shorter fixed ends can be custom ordered, but some companies have long waits for those (weeks or months) because the straps are made in China. A U.S. based company can provide faster service.
You asked, so I'll answer but you do not want to use us as an example. We have way-more straps than we need for most loads, but we have never run short of straps in front of a customer either. We also use more straps than are needed to sufficiently secure the load, especially if the customer is watching us load and secure the freight. If two straps will do, we'll use four. Doing so generates positive comments on the spot and positive feedback to our carrier when customer comments are offered. To us ratchet straps have two uses: 1) securing freight. 2) marketing.
We carry 12 yellow, 16 gray and 4 blue, plus 8 custom-made yellow straps with shorter ends.
In your case, it might be best to start with the minimum and grow from there as your loads and needs and budget dictates. Unless you are handling odd-sized and shapped freight on a regular basis or very expensive freight, you will not need anywhere near the number we use.
Examples of such freight include a room full of computer servers and associated equipment, none packed before your arrival; or 47 pieces of hand-made, high-cost, store fixtures, all brand new, not a scratch
on any of them and people watching you load and unload at both ends.
Finally prices for ratchet straps vary greatly by vendor. If you enjoy tossing your money in the street, go to a truck stop and buy your straps there. If not, most trailer shops sell them at a competitive price. You can also order quality straps from New Haven Moving Equipment. Another quality maker of ratchet straps is Kinedyne.