An unusually large number of Prince George's County police officers, including five in higher ranks, are seeking retirement in the next few weeks because of changes in pension and disability retirement regulations.

Although the total of 17 seeking retirement is a tiny fraction of the 859-member force, the vacancies will give the county's new police chief opportunities to reshape the department. A new chief is to be named by County Executive Lawrence J. Hogan to replace Chief John W. Rhoads, who himself is retiring Friday on a disability pension.

One of those planning to retire is Lt. Col. John Magruder, a 30-year veteran who was in the running for the chief's job when Rhoads got the appointment in 1974.

Others include Maj. George Steffey, Rhoads' executive assistant and close friend; Capt. Edward Armstrong, Rhoads' chief aide; Capt. George Connor, head of training, and Capt. Daniel Robertson, head of research and planning.

The number of retirements is "the most we've ever had . . . at one time," according to Laney Hester, president of the county's Fraternal Order of Police, the policemen's union. He attributes many of the retirements to a new pension plan that goes into effect Sunday allowing police officers to retire after 20 years instead of 25.

Six of the 17, including Rhoads, plan retirement on disability pensions. After Saturday, officers with more than 20 years' service will be barred from requesting disability retirement.

Under the disability retirement formula, an officer receives 70 percent of his salary if disability is service-related, 50 percent if it is not. This compares with a regular retirement formula of 48 percent after 20 years, adding 2 percent for each additional year worked to a maximum of 62 percent.

Four of the retiring senior officers are seeking regular benefits. The fifth, Armstrong, claims a service-related disability - a back injury.

Disability retirements have stirred disputes between budget-conscious county officials, who apply strict standards to defining "disability," and police union officials, who say their men are losing benefits to which they are entitled.

A medical board reviews disability retirement requests, then a review board issues a final ruling.

Four of the officers seeking disability benefits filed their requests after Rhoads made his well-publicized pension request last month.

The requests brought a sharp response from County Council member Sue V. Mills, a persistent critic of the retirement system. "The chief got away with it, why can't they?" she said. "Political jurisdictions are not going to be able to pay people not to work."