As an organiser, you can add multiple event-level giveaways. You can choose between:
Having one winner per giveaway, so for example, 5 giveaways = 5 winners, or
Having it bundled so all giveaways end with only one winner (or whatever quantity you choose), so 5 giveaways = 1 winner.
Giveaways can either be provided by some (or all) of your speakers or; alternatively on an event-wide basis (i.e. unrelated to the speakers).