I think that what is "too much" depends on what a client needs to feel a connection to the therapist, what a therapist is comfortable revealing, and how helpful it is in a client's treatment.

For example, if a therapist reveals stuff that damages the relationship, then it was probably too much, or inappropriate.

If a client needs to know a little about a therapist (relationship status, whether or not they have kids, some information about their interests) then that's not too much.

My T has not disclosed very much about herself, and when she has, it's either because I've directly asked something, or because she thought it would be helpful in illustrating a point. She's never not answered when I've directly asked something, but she will sometimes give vague answers. She's freely offered that she's married, has one son, a dog, and lives in the same suburb I do. I've picked up other things about her just based on conversations we've had. When I asked which winter holidays she celebrated (so I could wish her a pleasant one), she told me her religion. When I asked where she went on vacation, she shared that with me. It's always been small talk kind of stuff, though. The only time she's disclosed something personal without me asking was in the context of illustrating how screwed up my FOO is...she briefly mentioned something about how she relates to her son vs what I grew up with. I took it in and told her to tell her son he's very lucky to have her for a Mom, and then we moved on.