Standard package is the most popular package but premium package is for those clients who are looking to work the best of the best.
All professional designers are allowed to participate in standard and premium packages and there is limit of max 10 designs. But the limit is extendable to unlimited designs at the discretion of the client. Every time a contest holder gives a star rating of 4 or 5 stars on a design, designer's design limit will be increased by 1. So if you give 4 or 5 star rating on a designer's 3 designs, their limit will increase by 3.
However, the number of expected design submissions vary for each package. The top designers prefer the premium package due to high package amount.
You can check various packages here.