At least 34.2 million Americans in 1984 were receiving monthly public or private pensions earned in a job, and monthly benefits averaged about $590 per person, the Census Bureau reported yesterday.

But even those numbers don't give a complete picture. Many people receive more than one pension, and 10 million pensioners -- such as children and Americans living abroad -- were not included.

About 55 percent of the 34.2 million recipients of job-related pensions were women, a large number of them widows living mainly on Social Security.

But many others were younger people, notably career military personnel who retired in their 30s and 40s after 20 years of service and whose families averaged $3,558 a month in total household income from all sources.

The figures were derived from the bureau's new survey of income and program participation, the most detailed in its fact-finding collection. It found that:

*About 30.2 million people in the third quarter of 1984 received Social Security old-age or disability benefits based on their own employment or that of a spouse. 7.9 million people received private pensions.

*2.9 million were getting pensions from state and local governments and 1.8 million from the federal government.

*Pensions based on a military career totaled 1.3 million.

*Federal railroad retirement pensions went to 781,000.

Eliminating overlap where a person received more than one pension, the 34.2 million total meant about one pensioner for every three working men and women.

But officials said the figure understates the actual number of pensioners because the survey measured pensions derived from employment in the labor force and did not include certain recipients of welfare and veterans' benefits. Moreover, the survey covered only nonfarm, noninstitutional U.S. residents 25 and over and thus excluded children on Social Security and certain other groups such as pensioners living abroad. If all these groups were included, the total would be about 10 million higher.

The survey found that in the 34.2-million adult group receiving job-related pensions, the average was $590 per month. But the average was much higher for men ($780) than for women ($440). In every category women's pensions were lower, because, the survey said, women generally work fewer years and in lower-paid jobs.

Millions of persons were receiving more than one pension. More than four-fifths of private pensioners also received Social Security, for example. But, depending on a number of circumstances, the combined benefit could be more or less than the average.

Thus Social Security payments averaged $410 a month per person and private pension payments $360, but people getting both averaged $830 a month.

Similarly, state and local pensions averaged $530 a month, but those who also received Social Security had a combined income of $930.

Federal government pensioners averaged $920 a month, but those with Social Security received a combined average of $1,056. Military pensioners received an average of $1,000 a month, but those getting Social Security had a combined average of $1,480.

Most pensioners were elderly. Including the military, only 17 percent of all the 34.2 million pensionere were under 62. But of those receiving military pensions, 49 percent were under age 55 and 86 percent of that group were working, reflecting the career military's "20 years and out."

Looking not at individual income but at total household income of pensioner families -- including income of all persons in the household from pensions, work and other sources -- the study found military pensioner families the best off. Retired military households averaged $3,558 per month, reflecting both high pensions and continued employment after retiring.

Total income of households with federal government employe pensions was $2,253 a month; of households with state and local government pensions, $2,301; of households with private pensions, $1,955. But about 20 million of the 30 million Social Security pensioners had no other pension, and this group's total from all sources was $1,598 a month.