is it possible to create a smart folder where all emails from one recipient and the emails sent to the same recipient are displayed?
Yes sure. It will be a Search Folder.
If Search Folders are not already visible, you can enable them in Menu > Settings > General > General > Show Search Folders. Then right-click on the Search Folder and choose New Search Folder. Give it a name, and then enter the person’s email address in the From and To fields.
Another option is already provided in the Contact Details sidebar, which shows a history of all correspondence with the person. Click on the contact icon in the lower right of the UI to show the Contact Details. Then when you click on any email in your Inbox for that contact, all their email history with you will be displayed in the sidebar.
but if i use From and To together emclient find only a small part of the emails
only From: finds more
is it possible to display the number of unread mails next to the smart folder?
Unfortunately not. Search Folders do not show an unread count in the folder list like other folders do.