The actual number of points needed is on the website and best to check in case it's changed. Availability is pot luck and you just have to ask them, only a certain number of seats are available for points on each flight. So, if WT+ has (say) 24 seats, they might only allow two or four to be used with points. If those places are already booked then the flight is effectively 'full' for you. Upgrading with points is the most cost effective use of them.