First: start making backups! You will need the backup one day.
Yes, you can lose data. No, the upgrade is designed by Apple to be as careful as possible in a normal situation. By normal I here mean that you save your data and documents in the normal places, that is under
/home in your user folders or in folders you have created yourself directly under
If you have data in the "OS" folders, say under
/bin and so on, don´t expect that data to survive an update. The update process will do its best, some files will be copied to a special folder, but some files may be replaced and your data lost.
And as other answers mention, your applications may not survive a new OS version. You might need to upgrade the applications as well in order to read the data. And there might not be newer versions available or they may come at a cost.