Air carrier Captain on arrival into PHL voiced concerns about clearance complications that resulted from multiple flights with similar callsigns.

Narrative

Inbound to PHL; while on Washington Center frequency; we observed numerous similar call signs; all of which were the same company flights. Our flight number was ABC. There were a minimum of 7 similar call signs on the same frequency; also apparently inbound to PHL. ATC began issuing holding instructions to the aircraft inbound to PHL. Along with holding instructions; ATC was issuing numerous altitude change clearances as aircraft were entering and exiting holding. ATC frequently repeated clearances; due to the number of similar call signs; further congesting an already highly-congested frequency. Cause; the department responsible for assigning flight numbers did not adequately consider the safety implications of numerous similar call signs. Suggestions; the assignment of flight numbers is a function presumably performed within marketing or another department other than flight operations. I strongly believe that flight operations should provide input and coordinate to preclude assignment of similar call signs. The hazard identification and risk assessment process under SMS would be extremely useful in addressing this issue.