When we apply identity key constraints then only Clustered Index will be automatic generated. At the time of table creation only Non-Index Cluster will be available. A table can have only one Clustered Index and 249 Non-Clustered Indexes.

Clustered Clustered indexes sort and store the data rows in the table or view based on their key values. These are the columns included in the index definition. There can be only one clustered index per table, because the data rows themselves can be sorted in only one order. The only time the data rows in a table are stored in sorted order is when the table contains a clustered index. When a table has a clustered index, the table is called a clustered table. If a table has no clustered index, its data rows are stored in an unordered structure called a heap.