As an oracle do they only take trance of one deity?
Is there a limit to the number they can take trance of?
Can any deity be chosen?
I believe the exact number is dependent on the deity/deities in question, the oracle's "ability" to withstand the extreme physical pressures required while taking trance and certainly the Guru's guidance. The oracles are, after much purification and training, perfect vessels for deities but who enters, depends once again, solely on the Guru's discretion. This may not be so apparent of course as the Guru after all embodies humility.
Some deities are more wrathful than others.... take for example, when an oracle takes trance of Lord Setrap it is often quick as he is most wrathful and the energy required to sustain a being like that is massive. Even our King Dorje Shugden, depending on the form that is invited, is often rather wrathful... which is why in most cases when answers are presented to Our King through the oracle taking trance, Our King stays for only a short while, leaves and then sends his minister Lord Karche Marpo to answer the questions.
I have been very blessed and privileged to be able to witness this incredible phenomenon on a couple of occasions which cannot really be described in words. Even watching a video of it cannot compare to actually being in the presence of such divine energy, elaborate customs, ritual, flair and mysticism.