The recommended way of installing a package on Arch, even a single package, is do it on a fully updated system because partial upgrades are unsupported.
So; sudo pacman -Syu <pkg>
This will --sync, --refresh your pacman databases, --upgrade your already installed packages and will install <pkg>.
If in this process there is any package that needs rebooting when upgraded or installed, you will be prompted.
I remembered that I had seen this thread on this forum. There is a post with an script which checks if there is any packages that require rebooting if upgraded and informs you if such is the case.
If you are in the middle of some important work that cannot be interrupted for a reboot and feel a sudden urge to upgrade and/or install a package, run that script before to know what will be coming for not getting surprised by a nasty reboot prompt 