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does that mean you have to play more than one instrument?
In short, no, but you will be expected to have keyboard studies to about the level of Grade 5, though you need not have taken the exam - this is because of the general musicianship skills you will study throughout your first and second year. Keyboard skills are normally included within the audition process.
I do know of a composition student who was offered place this year without much experience on the keyboard - he composed using the software Sibelius etc. - but they expected him to have acquired sufficient skill by the time he started the course, otherwise they simply wouldn't have let him in.
A well-rounded musician would be someone who can demonstrate practical performance skills in various situations, such as solo, a small ensemble, chamber music and within a large orchestra where your own input as well as working as part of a team is important. Then there's teaching skills, work with students of all ages and backgrounds and communicating your subject and skills to a wide range of audiences. All the major UK conservatoires now run outreach groups that students are expected to participate in at some time and these will help develop your communication skills. Sadly, there is insufficient work around to just perform these days and even established orchestral musicians teach in order to supplement their income.
Best advice is for you to gain as much experience as possible performing with others (preferably in public), before you apply to the RAM, so that you can put all of this down on your application form. The more practical experience you have, the better your chances of being selected for audition.