A fact of life of being a freelancer is that you can’t say for sure where your income will be coming from three months from now. One of the reasons that companies hire freelancers is because they don’t have a commitment to them as they have to their employees. When times get tough usually the first people to be shown the door are the people working on contract. It’s very comfortable to be working for one client. You may not however feel so good when your entire monthly income comes to a sudden halt without any severance pay or possibility of receiving unemployment insurance. In a similar vein you may not see a problem if you’ve been getting new clients from a sole source such as word of mouth or through one type of advertising . However when the stream of new clients start to dry up keep in mind that it may take a while to establish a new avenue for bringing in new clients.
My suggestion–keep working for one big client by all means, but don’t shun your returning clients if they come back to you for additional work, even if it means working 15 hour days for a short period, as you may need these returning clients when your current one big client lets you go. Also, if a new client approaches you, you should carefully consider taking them on if your prior commitments allow you to. It may be a bit of a burden to you, but small projects often blossom into bigger ones and you may need a big project in the future. Do not however take on so much work that both the new client and the existing large client are unhappy with your work. Only you know how much work you are capable of doing.