Setting the value of a column to an integer one greater than the number of rows in the table is risky, as it does not cater for any breaks in the sequence arising from the prior deletion of one or more rows. To increment a sequence of numbers by 1 the usual method is by setting the value for a new record to 1 greater than the highest current value in the table.
You might like to take a look at CustomNumber.zip in my public databases folder at:
This little demo file illustrates how to assign a sequential value to a column when a new row is inserted by means of a form. It caters for possible conflicts in assigning the value if two or more users are inserting a row simultaneously, and allows the next number to be used to be seeded.
The method illustrated in the demo assigns a value to a primary key column, so if any rows other than that with the highest number have been deleted, a gap will be left in the sequence. The demo also illustrates how to fill such gaps if desired. As the sequence will the have no semantic significance, however, it would be better to return the sequential numbers at runtime in a query. RowNumbering.zip in the same OneDrive folder illustrates a number of queries which do this.