Is there any best number for rows count in a table to start using indexing? I'm a newbie in DBA. I want to know if there is any specified point in a table's rows count that when we reach that point, then we could use an index. I know there should be some dependency between table's rows count and using index, but I don't know if there is an standard table's rows count or not.
An index is an on-disk structure associated with a table or view that speeds retrieval of rows from the table or view. An index contains keys built from one or more columns in the table or view. These keys are stored in a structure (B-tree) that enables SQL Server to find the row or rows associated with the key values quickly and efficiently.

I want to know if there is any specified point in a table's rows count that when we reach that point, then we could use an index.

The rowcount in a table does not dictate the use of Index. Its your queries (pattern) that should dictate the need for creating indexes, so that the data retrieval is as fast as possible.
The query optimizer relies on statistics to produce optimal query plan.
From SQL Server Index Design Guide :

The selection of the right indexes for a database and its workload is a complex balancing act between query speed and update cost.
Narrow indexes, or indexes with few columns in the index key, require less disk space and maintenance overhead.
Wide indexes, on the other hand, cover more queries. You may have to experiment with several different designs before finding the most efficient index. Indexes can be added, modified, and dropped without affecting the database schema or application design.

You should use SQL Server's DMV to tune your indexing strategy or even use sp_BlitzIndex to get more insights.
Refer to :

Query Tuning Fundamentals: Density, Predicates, Selectivity, and Cardinality
Ten Common Threats to Execution Plan Quality