Question: Is it a good idea to only learn from one Dharma teacher?
Answer: This is alright only if that teacher is the Buddha, a perfect teacher! However, as there is no Buddha present at the moment, with only the Buddhist scriptures (经) around, that might not be easy to interpret personally, there should be learning from human teachers for their true meanings. With reference to the relevant Patriarchs’ explanations, there should be learning from those who allow and encourage active enquiry, not those who demand blind faith only.
As human teachers are non-Buddhas, thus imperfect, it is safer, for more correct and comprehensive learning, to learn from at least a few specialised and qualified teachers, before perhaps settling on a main teacher or two. With this cross-referencing, we can better ensure that there will be Right Understanding (正见). Thus, those who insist there should be learning only from himself or herself without further question are those who should be stayed away from, as they might be arrogant and thus deluded, far from enlightenment.
Question: Is it a good idea to only learn one Dharma teaching?
Answer: This is alright only if that teaching is exactly the one most suitable for you. However, it is with adequate general learning that you will know what teaching to best focus on. Of course, as life is short and unpredictable, there should not be too much time expended on ‘random’ exploration. If there is uncertainty, it should be noted that the Buddha taught that the Pure Land teachings are most feasible in this Dharma-Ending Age (末法时期).