How do I use ROW_NUMBER()?

For the first question, why not just use?

SELECT COUNT(*) FROM myTable 

to get the count.

And for the second question, the primary key of the row is what should be used to identify a particular row. Don't try and use the row number for that.

If you returned Row_Number() in your main query,

SELECT ROW_NUMBER() OVER (Order by Id) AS RowNumber, Field1, Field2, Field3
FROM User

Then when you want to go 5 rows back then you can take the current row number and use the following query to determine the row with currentrow -5

SELECT us.Id
FROM (SELECT ROW_NUMBER() OVER (ORDER BY id) AS Row, Id
     FROM User ) us 
WHERE Row = CurrentRow - 5   

Though I agree with others that you could use count() to get the total number of rows, here is how you can use the row_count():

1. To get the total no of rows:

   with temp as (
       select row_number() over (order by id) as rownum
       from table_name 
   )
   select max(rownum) from temp

2. To get the row numbers where name is Matt:

   with temp as (
       select name, row_number() over (order by id) as rownum
       from table_name
   )
   select rownum from temp where name like 'Matt'

You can further use min(rownum) or max(rownum) to get the first or last row for Matt respectively.

These were very simple implementations of row_number(). You can use it for more complex grouping. Check out my response on Advanced grouping without using a sub query

If you need to return the table's total row count, you can use an alternative way to the SELECT COUNT(*) statement.

Because SELECT COUNT(*) makes a full table scan to return the row count, it can take very long time for a large table. You can use the sysindexes system table instead in this case. There is a ROWS column that contains the total row count for each table in your database. You can use the following select statement:

SELECT rows FROM sysindexes WHERE id = OBJECT_ID('table_name') AND indid < 2

This will drastically reduce the time your query takes.