The amount of photos that will fit on a 128 GB flash drive will depend on the size of the photos as higher resolution photos are bigger in data size than lower resolution photos.
But even with high resolution photos you can store a ton of those photos on one single 128 GB flash drive which is amazing.
Assuming your photos are 3 MB in file size that 128 GB flash drive can hold roughly 44,690 photos which is a very good large amount of photos to store.
Data will typically stay on a flash drive for a period of about 10 years which is quite a long time and after 10 years the electrical charge in the flash drive dissipates which can mean a loss of data on the flash drive.
A flash drive may hold the data longer than 10 years but after 10 years the flash drive is not as reliable at saving information.
Make sure you backup the data on a new flash drive before 10 years to be safe and have the backups of the data on other things as well.
Flash drives are a wonderful storage device as they are easy to use and move around and can hold a lot of data for the size of the flash drive.
Flash Drives are pretty reliable for storage but after about 10 years of the data being on the flash drive it's possible for the flash drive to lose the data or the flash drive become unreliable to connect too.
Flash drives store data through electrical charges and overtime the electrical charges on the flash drive dissipate which means that the flash drive can cause the data to be lost.
So if you have data on a flash drive make sure to get a new flash drive before 10 years and transfer the data over to the new flash drive to keep from losing the data.
Ideally you should have the same data backed up on your computer hard drive, additional flash drives, external hard drives and in the cloud storage.
If the data is really important to you such as pictures or databases that you cannot replace back the data up on multiple devices so you have them readily available.