It would be beneficial to allow the admin/agency manager to choose the option of letting people choose multiple shifts or being able to limit to just one shift. That way people do not double (or sometimes triple) book. This is most benefitical when you are trying to get people to commit to either half day shifts in the morning, half day in the afternoon, or all day.
Thanks for your suggestion about shift restrictions. To clarify, are you saying that you want to be able to prevent a volunteer from signing up for, say, both the morning shift and the afternoon shift? Would you want this to be a site-wide feature (where the site manager decides whether the restriction can apply to all needs) or need-specific (where the agency or program manage makes that choice)? Would either work for your use case?
Thanks for posting!