If I purchase Lightroom 6 can I install it on my desktop and my laptop ie. are there 2 licences with the 1 purchase.
Yes, for home use, anyway, installing on both a desktop and a laptop is specifically permitted under Adobe's End User License Agreement for Lightroom, as long as Lightroom is not used on both machines simultaneously:
Portable or Home Computer Use. The primary user of the Computer on which the Software is installed may install a second copy of the Software for his or her exclusive use on either a portable Computer or a Computer located at his or her home, provided the Software on the portable or home Computer is not used at the same time as the Software on the primary Computer.