The installation of Nebulb requires between 250 MB (for the version without GPU acceleration) and about 2 GB of disk space (for the version using the graphics card).
However, during the image stacking process Nebulb needs a temporary working folder where it stores all the progress it makes.
Thanks to this, Nebulb can stack thousands of images without any problems and, in addition, there is no problem if the computer power goes out or if you decide to pause the stacking process (later you can resume where you left off).
And as a general recommendation about the temporary working folder: it is interesting that the disk where it is located has, at least, between 100 and 200 GB free (although this is something quite flexible because Nebulb adapts to the available disk space).
But this does not mean that Nebulb will always use all that disk space, simply that in the case that a lot of images are stacked Nebulb will have enough free space to be able to do its work properly.
In addition, when Nebulb finishes stacking and you save the project, all the disk space temporarily used is automatically freed.