You can opt in to as many Data Extras as you like. However, some Data Extras will be used up quicker than others. For example, usage of a 24-hour Data Extra will always take priority over one that lasts a full month.
The easiest way to monitor your data use and to buy data extras is by downloading and using the My Vodafone app. If you find you're exceeding your monthly data allowance regularly, it may be more cost effective to move to a plan with more data.