That is a tricky question.
If you just saving BIAB songs, without exporting individual WAV stems to be saved to the same "song" folder (I do that), a 3-5 GB should be all you need, which should last you long time. You can always move that folder, when you upgrade computer or storage.

For easy access, I made a folder "BIAB projects" on desktop, where I save arrangements from BIAB.

But to answer your question more directly, there should be enough space for reasonable amount of songs saved on that external HD with Ultrapack.

P.S. If you are exporting individual WAV stems for DAW or such and saving to same folder, space required would depend on your usage.