The gain in design efficiency tapers off pretty quickly as you increase the number of versions you have, but you have an added benefit in preventing order bias by generating multiple versions of the design. In SSI Web, most of the time it's going to take an extra few seconds to generate 300 versions instead of 30, so it becomes more of a "why not?" type of thing.
Honestly in my opinion, having 20 unused versions probably won't cause any harm, but an 80-version design should be quite satisfactory as well. The other answer here is a bit misleading, because each version of a design is constructed to be balanced and orthogonal. A lot of the time people are using a single design from a book or library of designs, so you don't "need" multiple versions, it just carries some benefits we think.
It probably is the "safer" choice to have 80 and repeat some, just so your design tests reflect perhaps your worst case scenario (indeed you should always be testing the design with your expected sample size/worst case scenario sample size).