You can instantiate a StandardSetController in either of the following ways:

1. From a list of sObjects:
List<account> accountList = [SELECT Name FROM Account LIMIT 20];
ApexPages.StandardSetController ssc = new ApexPages.StandardSetController(accountList);

2. From a query locator:
ApexPages.StandardSetController ssc =
new ApexPages.StandardSetController(Database.getQueryLocator([SELECT Name,CloseDate FROM Opportunity]));

Note:
The maximum record limit for StandardSetController is 10,000 records. Instantiating StandardSetController using a query locator returning more than 10,000 records causes a LimitException to be thrown. However, instantiating StandardSetController with a list of more than 10,000 records doesn’t throw an exception, and instead truncates the records to the limit.

Cheers!!!