Does upgrading macOS causes one to lose data? I am currently running macOS Sierra 10.12.6 on my Mac. I am in the process of upgrading the OS to the current public release macOS High Sierra 10.13.6. Will I lose my data during the upgrade process?
No.

Generally speaking, upgrading to a subsequent major release of macOS doesn't erase/touch user data. Pre-installed apps and configurations too survive the upgrade. Upgrading macOS is a common practise and carried out by lot of users every year when a new major version is released.

However, you should be cautious and be responsible for the sanity of your data. Upgrades doesn't negate the possibility of going wrong, and in the unlikely case, may cause damage to the hard drive, thereby causing you to lose access to data.

It is highly recommended to keep properly working data backup when doing a sensitive activity like OS upgrade.

I'll put extra emphasis on having a backup that works! A backup that doesn't work is as good as no backup.

Ensuring you have proper power backup and uninterrupted Internet connectivity, you should be fine with a high degree of probability. Having a backup ensures complete peace of mind :)