You can attach multiple files to email at once easily, no matter if you use Outlook, Gmail or other email provider.
Adding multiple attachments to email at once:
Windows PC - click on Add attachment icon, to open the file explorer. Hold the Ctrl key and click on files you want to attach. After you click on Open, all selected files should be attached at once.
Mac PC - click on Add attachment icon, to open the file explorer. Hold the Cmd key (Command key) and click on files you want to attach. After you confirm the selection, all selected files should be attached at once.
If there are many files you need to attach, you can hold the Shift key and select the range of files. You can select the range of files with 2 clicks, instead of clicking on each file.