As a general rule, no one customer should account for more than 20-25% of your revenue. While having large customers can be good for your bottom line, it introduces risk to the next owner.
As you build your business, pay attention to what a future buyer will want. They’re looking for well-diversified customer groups where the loss of one account won’t have a devastating impact. Do the hard work of diversifying, and you’ll increase your future business value.