Memory sticks are quite cheap these days and even a cheap one (I spotted a £6 one for 8GB in my local supermarket yesterday) will probably hold them.
But to be sure, I would look at the size of the folder holding the photos and see how big they are. If you're using Windows (?), open My Computer / File Explorer/ This PC and right-click on the folder holding your photos to see what the Size is?
However I would also advise having more means of backup - if your memory stick failed, or your computer's hard disk, then you wouldn't want to lose your only copy. Memory sticks are OK but I wouldn't rely on them totally, especially a cheap one. So as well as keeping a copy on a memory stick, I would consider burning the photos to a DVD, or using a portable hard drive or (if you have a good broadband allowance) copying them to an online site in 'the cloud'.