You would need a thumb drive that is at minimum of 8GB?. This is the smallest size you should consider acquiring. I wouldn’t pressure yourself into getting anything larger as you can only load one OS? at a time on the thumb drive. There are tools like Ventoy, but that is beyond the scope of this tutorial. If you have to buy a new thumb drive, keep in mind of the ports you have available. If you have just USB-C ports, then buying a thumb drive with USB-A is pretty useless.
If you can’t find 8GB thumb drives, that is not an issue. This is the minimum, and if you could only find 32GB drives, those will work too. Again, you can only have on OS on a drive at one time. If you aren’t sure what brand to choose from, I always found Sandisk to be a good brand, and never had problems with them. Again, your more important issue is the 8GB capacity.