Is it possible to install just the mongo Shell?

You can use HomeBrew, see the official MongoDB HomeBrew page.

Just add the tap:

brew tap mongodb/brew

And then after adding the tap from above, you can then install the Mongo shell with:

brew install mongodb-community-shell

You can download the latest Mongo Shell for MacOS from the following URL:

https://downloads.mongodb.org/osx/mongodb-shell-osx-ssl-x86_64-3.6.2.tgz

For the sake of completion, the Mongo Shell for Windows is available at https://downloads.mongodb.org/win32/mongodb-win32-x86_64-2008plus-ssl-3.6.2-signed.msi

Furthermore, for other Linux distributions, I suggest you create a MongoDB Atlas cluster (for instance, a free M0 cluster) at www.mongodb.com/atlas where you'll find the Mongo Shell downloads for all OSes in the Connect dialog of your cluster.

I hope this helps.