The price is for LR 5.anything so a single serial number works for all the LR 5.something versions. When LR 6 comes out, presumably next year, it’ll be a payfor upgrade from LR 5, unless you have purchased the Photography Plan, then it’ll just be the same monthly price.
For perpetually licensed versions:
UpDATES are free e.g. 5.6 to 5.7.
Updates usually include new camera support and bugfixes.
UpGRADES cost money e.g. 5.7 to 6.0
Upgrades include new camera support, bugfixes plus new and enhanced features.
For Cloud versions, all updates and upgrades are included in the monthly fee. No extra cost involved.
Bear in mind, that Lightroom Mobile is only included with the subscription version. Lightroom Mobile is not included with the perpetually licensed version.