Currently, a new version of QGIS is released around every four months. The version released each spring is designated as a long-term release (LTR). This means it will be supported for one calendar year. Each quarter, a new stable version is released and bug fixes applied to the LTR. The LTR is recommended for production environments since it has a slower release cycle. At the time of writing, QGIS 3.4.5 is the current LTR, with version 3.6 available for download.
It is possible to have multiple versions of QGIS installed on a single machine.