Agency staff are also staying longer in their jobs. The census shows that staff turnover last year dropped by 4.3 per cent compared with 2006.
But although turnover has stabilised at around 20 per cent, the IPA warns the figure could rise this year as a result of the new Tupe regulations.
The increase in employment levels can be explained partly by the election of three new agencies to IPA membership. However, the census reflects a trend that has seen industry numbers growing steadily since 2001, when they stood at just over 14,000.
Hamish Pringle, the IPA director-general, said: "I suspect that, to some extent, the rise is due to agencies staffing up in the digital area. Also, the size of the market is growing as search becomes a powerful category for the business."
Agency staff remain almost equally split between men and women. However, almost 84 per cent of agency chairmen, chief executives and managing directors are male.
At the same time, the agency workforce remains overwhelmingly young. Almost half is under 30 and little more than 5 per cent over 50.
Nor is there much evidence that people from ethnic minorities are either being attracted to careers in agencies or being employed by them.
Figures based on information supplied by 67 IPA agencies showed that almost 94 per cent of their staff are white.
* Just over 19,000 people were employed by 261 IPA agencies last year - a rise of more than 1,700 on 2006.
* The average age of agency staff is 33.5 years.
* The workforce is split almost equally between men and women. But only 16 per cent of senior agency managers are female.
* Just over 2 per cent of agency staff are Asian and just 1.7 per cent are black.
* Almost half the 1,179 temps and freelancers working for agencies are creatives.
* First-year trainees account for just over 3 per cent of the agency workforce.
* Just over 13,500 people work in 205 creative, full-service and other non-media agencies. Just over 5,300 staff are employed by 56 media shops.