You could either select one of the images in the folder, then press the key combination ⌘A to select all,
or simply select the folder and then compress the folder as described above.
Will I be able to send the zipped files in one email?
That will depend on the size of the compressed file - see Aye Es Oh's post. Zipping images does not compress them much. Look at the size of the zipped files; if it is much larger than 3 MB the recipient of the mail will not be happy.
If you want to compress the images to share them by mail from Aperture, you could use Aperture's Mail preferences in the "Export" panel to set the size of the mailed images to small. Then you should be able to mail 20 images at once.
Regards
Léonie