Whether you need local or national SEO depends on the type of business you have. Local SEO services are ideal for getting more customers into a venue, for example a bar or restaurant, or for being found by customers when providing a service in a single town or city, for example for a sole trader or a repair person.
National SEO is for companies who provide country-wide or international services and sales. These require a different approach since they need to reach a much wider customer base. Typically if your business sells goods of any sort online, a national SEO plan is the way to go.