Maybe one or two. Since you are only using them to temporarily hold the files that you are transferring from one computer to another, one flash drive is all you really need.
Try to keep your files in one centralized and organized location. Using a "Cloud Drive" is one method to centralize your files. Another is to use your computer and then backup to an external hard drive.
Never use your USB flash drive to store your primary files on, or to backup to. They have what is called "volatile" memory, meaning erasable. One day you may find your files all gone. Flash drives have a limited lifespan, said to be up to 10 years, but in practice, probably closer to 5 years. The more you use the USB flash drive, to read and write files, the shorter the lifespan. The older they get, they are more susceptible to to file corruption, which you may not recognize right away.