Anyone know whether there will be more buses or less buses during Summer months ?

Is it necessary to book ahead and can I just get to their station for the current ticket ? Will it be full usually ?

They run every 15 minutes duting the day, every 30 in the evenings. During Edinburgh Festival can be very busy and Summer weekends, when big events are on etc... if you can't get on one bus they just stick you on the next one. I've used them quite a lot (including 5 weeks of commuting 6 days a week) and they are pretty good.
Think it can be cheaper if you book in advance inline, but not always.

there are regular trains running as well,every 15 minutes during the day,about £3 or £4 more expensive than the bus,but roughly to and from the same area of the 2 cities.

There are more buses on Sunday.... I mix and match train and bus depending when I need them....

You can also book through Megabus - though the bus you end up on may be a Citylink one