A number in the plan name specifies the max allowed number of people on the guest list. In this case, the plan '100+' has a limit of 100 people. The '+' sign is used to distinguish it from the free plan '100' which also allows to add up to 100 people.
To host more than 100 people you need to upgrade to plan '200' or more depending on the number of people you are expecting.
You can upgrade to the next plan at any time and would need to pay only the difference in plans' prices and add-ons if any.