We have to define here what is meant by "have a teacher" here.
I have someone I consider a root Guru, and two other teachers who I listen particularly closely, one of whom I have a personal relationship, and one I don't, simply because they have lots of students.
However, I've taken teachings from scores of teachers. I don't consider the scores invalid, I just have a much more defined relationship with the first category.
I mean "having one teacher" could literally just mean one teacher with whom you have a personal relationship. You could listen to them to the extent that you actually refuse to even read other teachers. This approach is out there, personally I think it's bunk.
On the other hand, you could have one or a handful of people you consider authorities that really speak your language, literally or figuratively. So you read other stuff, and are mainly responsible for your own practice, checking in with teachers when you can or need to. I opt for this relationship personally.
Here is some advice I read years ago from Tenzin Wangyal that is very practical in my opinion:
Tenzin Wangyal Rinpoche: In order to mature on the spiritual path, it is important to have a teacher with whom you make a heart connection. When you receive instruction from that teacher, you need to take those instructions into your life and practice them until you have experiences and realization. This process requires trust, love, and commitment.
There are individuals with enough realization, and more important, stability, who can be exposed to many ideas and process them without weakening their commitment and the stability of their practice. If someone is open, capable, and stable, then it is fine to learn from different teachers and traditions— and areas outside of Buddhism, such as science, anthropology, and philosophy. But while there is no limit to what we can learn from others, it is important to narrow down the number of practices you engage in, because practice requires long and deep commitment in order to bear fruit.
It is important to explore what’s motivating you to leam from many traditions and teachers. Listen deeply to discern if you are in search mode, driven by an underlying feeling of dissatisfaction. Without realizing it, we can spend years in this mode, wandering and collecting knowledge, all the while not connecting with our underlying hunger. If this is the case, we are not really trying to find a teacher or to become more intimate with our actual experience. This is a form of spiritual materialism, and it is an obstacle to spiritual realization. If you look more deeply and honestly you may realize that you are lost, and this realization can be a genuine beginning to your path.
If this is the case, it is important first and foremost to find a teacher you respect and will come to love, and a path and practice you can commit to, then dedicate enough time with that teacher and the teachings and practices to develop confidence and maturity. During this time you can be open to others, not replacing or rejecting your master or your practice, but complementing, enriching, and expanding your life. However, if in listening to others you become confused, stop and concentrate on your path.
Confusion is the sign you must not ignore. First, we acknowledge our confusion in order to recognize the need for a teacher and a path, and later our pain continues to guide us to deepen our commitment and realization. Pain or insecurity is always the sign to stop and deepen your connection with yourself rather than to search outside yourself to fill what is missing.
As humans, we can love every human being, be inspired by many people, and have deep friendships with some. But we can’t build deep intimate relationships with everybody. If your main relationship is not deep enough, then having relationships with many people can create instability. For example, a husband or wife has spiritual and emotional and physical intimacy with their partner, and that person can also have emotional intimate relations with a small group or family. They can have true dharma brothers and sisters and a sense of emotional commitment, a feeling that they would do anything to help them and be present for them. But if you expand more than you are capable of, it causes confusion and you don’t reach your full capacity as a human. In the process of expanding, you may disconnect from those with whom you do have intimacy.
My advice is to listen with your heart and also analyze with your intellect to see if there is value in the criticism that you are not focusing solely on one path. Do you have a path, a practice, and a teacher you truly follow? If not, the first priority is to find that. Once you find that connection, you will be able to deepen spiritual intimacy through practice and commitment. Then, when you listen to other teachers who might have different views, it is not masking your confusion or adding to it; instead, it is enriching your life. But you must reflect and know your own capacity. Others cannot truly discern this for you. Only you can follow your heart and know whether you are in true relationship with your path. 0
It's worth noting, some of what Rinpoche talks about above can happen
within one tradition as well.