Indexes may be defined as unique or non-unique. A unique 
index acts as a constraint on the table by preventing 
identical rows in the index and thus, the original columns.

There can only be one clustered index but multiple non-
clustered index

One table can only one clustered index or primary key and
249 non-clustered index or unique key

In sqlserver we have two types of indexes (1) Clustered (2) 
non clustered indexes. 

Indexes may be simple index or composite index.

Index formed based on single column values is called simple 
index

index formed based on multiple column values is called 
composite index.

Default index is NON CLUSTERED INDEX 

When a table contains a primary key column then sqlserver 
automatically creates a unique clustered index.

Unique--- automatically creates a non clustered index 

so for this reason PRIMARY KEY IS NOT EQUAL TO UNIQUE + NOT 
NULL.

250 indexes(1 cluster index,249 non-cluster indexes)

four index keys is possible per one table.
read only table u havae create more then index per table