I have a client who has purchased several domain names relevant to her business. Some of them she purchased to prevent a competitor doing so.
Is there a limit to how many she should have pointing to the one website, even if they are all relevant?
There is no limit, per say, as long as the domains are just redirects and do not contain much duplicate content.
It can be good to build new sites on the empty domains, or a corporate blog or something else; provided of course, that you can justify creating new content. Often one web site is insufficient when promoting companies that offer more than one service or product, particularly when the services/products are unrelated. For example, I had a company that offered both eco-friendly and non-eco-friendly tourism related services. In all, I identified 5 separate business functions completely unrelated to each other thus decided on 5 web sites in order to promote it properly. In this case, I was able to influence my own off-site ranking factors by lightly cross-linking similar pages. This worked well for my client's rankings in a broader spectrum of keyphrase markets.