Mil-Std 105D suggests k=7 is a good number. Multiple sampling plans are usually presented in tabular form: 

The procedure commences with taking a random sample of size n₁ from a large lot of size N. and counting the number of  defectives, d₁ 

Efficiency for a multiple sampling scheme is measured by the average sample number (ASN) required for a given Type I and Type II set of errors. The number of samples needed when following a multiple sample  scheme may vary from trial to trial, and the ASN represents the average of what might happen over many trials with a fixed incoming defect level.