Hello!
I’m a sophomore, my GPA is a 3.9, I take a big schedule, and I am heavily involved orchestra, but not much else. I take an Honors Orchestra class, which gives me an academic hour every school day. I am involved in many groups and private lessons with end up taking about 4/5 hours a week, not counting concerts. I also practice around an hour every day. I play both viola and violin, and have gone to All-State Orchestra with viola, and one of my most prominent groups is my membership in a community symphony.
Would colleges be disappointed that my EC work is all in Orchestra, as much of it as it is, or would I be wise to tack on smaller commitment clubs to appear more well-rounded?
3.9 unweighted, if you’d like me to calculate weighted I could figure it out 
What else do you like to do, or have a sincere interest in? What do you do on your own when you are not studying, practicing your instruments or rehearsing?
I’m no expert in college admissions, but I would think one or two more ECs would be nice…but not as filler. They would have to be genuine. I think depth is definitely better than a lot of fluff, so what you are doing in orchestra is great, along with your great GPA! If time is short for you, do you have an EC (or job) in the summer that you don’t do during the school year? That would count too.
Also keep in mind, an EC doesn’t have to be part of a structured program…it can be an independent hobby or interest you develop on your own, i.e., an art or craft, collecting, inventing, developing a skill, helping others in some way… I have read some quirky but successful college essays about some of these independent interests that illuminated the individuality of the student. I f you have specific family obligations that you spend time on, that counts too.
Also, (if they follow true interests) I believe there are some short-term or seasonal school-based ECs (such as mock trial, envirothon, specific volunteer projects, etc.) that you might be able to incorporate into your schedule as long as they are genuine interests. You just don’t want to list a bunch of clubs that have no substance.