[A DROID]

For example if I get a samsung with android 5.0 OS will I be able to update it to android 6.0 OS and further or will it stop at some point?

[KCAndroid1]

All phones will stop updating at some point. Depends on the manufacturer/carrier, hardware specs, etc. Best sellers usually get a couple of updates. My AT&T Galaxy S3 launched on ICS, got Jelly Bean then KitKat but appears finished at 4.3. The Nexus line is supposedly good for at least 2 years from launch - Google has said the >2 year old Nexus 4 will get 5.0. Rooting can probably let you stretch that out, but that's a whole other topic.