A 32 GB USB flash drive or USB hard drive can hold between 3,000 to 4,000 pictures depending on the file size.
The higher the resolution of the photos or pictures the less of them the USB drive can hold and the smaller lower resolution pictures the more the USB drive can hold.
Still 3,000 to 4,000 photos is a lot of photos that a 32 GB USB hard drive or USB Flash drive can hold.
Also just to note if you're just storing the photos on a USB flash drive or memory card be sure to back the photos up to an external hard drive as well as on your computer hard drive and in the cloud storage as well.
USB Flash drives start to go bad after 10 years and the photos could be lost forever so always back them up before then.