My answer mirrors what others have already said, but what I can add is this: look at the top 10 customer concentration too. A customer at 15% of business can get rationalized, but seeing the top 10 customers over 50%, 60%, and 70% of the business is just as (if not more) concerning and can't be as rationally accepted.

I generally prefer <20% from any one customer, but I’d focus less on the threshold than on whether the concentration is manageable under new ownership. Why is there high concentration? Is it driven by seller relationships, a narrow sales engine, capacity constraints, or simply a few customers that grew faster than everyone else? Can a new owner realistically dilute it, how long would that take? I’d also ask what it would take to replace each major customer: how hard are they to replace, how long is the sales cycle, and how much revenue/profit would disappear in the meantime? Concentration that can be fixed through better commercial execution is very different from concentration embedded in the business model.