Job openings is a combination of job creation/industry growth, job turnover (replacements for people retiring or moving to different jobs), the economy and other variables.
A fast growing industry/job function will have less turnover (because more workers are young), so most jobs will be due to expansion, while an older stable industry/job will have more jobs from turnover and fewer from expansion.
These numbers vary by industry and job. I couldn't find any general formula to compute number of new jobs. The Bureau of Labor Statistics (BLS) reports job creation while O*NET OnLINE! reports need (creation + turnover).

Occupations with the largest job growth
Bureau of Labor Statistics (BLS) November 2007 Monthly Labor Review. Sorted by number of new jobs (thousands) in next 10 years.

Title	Jobs (1,000)		% increase		Sal- ary	Education or training
	2006	new	10 yrs	per year
High Salary
Registered nurses	2,505	587	23%	2.1%	VH	Associate degree
Postsecondary teachers	1,672	382	23%	2.1%	VH	Doctoral degree
Executive secretaries and administrative assistants	1,618	239	15%	1.4%	H	Work experience in a related occupation
Computer software engineers, applications	507	226	45%	3.8%	VH	Bachelor's degree
Accountants and auditors	1,274	226	18%	1.6%	VH	Bachelor's degree
Elementary school teachers, except special education	1,540	209	14%	1.3%	H	Bachelor's degree
Truck drivers, heavy and tractor-trailer	1,860	193	10%	1.0%	H	Moderate-term on-the-job training
Carpenters	1,462	150	10%	1.0%	H	Long-term on-the-job training
Management analysts	678	149	22%	2.0%	VH	Bachelor's or higher degree, plus work experience
Computer systems analysts	504	146	29%	2.6%	VH	Bachelor's degree
Network systems and data communications analysts	262	140	53%	4.4%	VH	Bachelor's degree
Maintenance and repair workers, general	1,391	140	10%	1.0%	H	Moderate-term on-the-job training

Specific jobs of interest to friends and family.

Title	Jobs (1,000)			% increase		Sal- ary	Education or training
	2006	new	need	10 yrs	per year
Biological Science Teachers, Postsecondary	1,672	382	662	23%	2.1%	VH	PhD
Medical scientists, except epidemiologists	87	17	45	14-20%	1.8%	VH	BS+
Dentist	136	14	39	7-13%	1.0%	VH	4 yrs of dental school
Chemist	84	8	30	7-13%	1.0%	VH	BS+
Biochemists and Biophysicists	20	4	7	14-20%	1.8%	VH	BS+
Dental Laboratory Technician	53	2	12	3-6%	0.4%	VH	Moderate-term on-the-job training
Biological scientists, all otherr	29	2	7	3-6%	0.4%	VH	BS+
Microbiologists	17	2	5	7-13%	1.0%	VH	BS+

new = new jobs created | need = (new + turnover)

Title	Jobs (1,000)		% increase		Sal- ary	Education or training
	2006	new	10 yrs	per year
Low Salary
Title	Jobs (1,000)		% increse		Sal- ary	Education or training
	2006	new	10 yrs	per year
Retail salespersons	4,477	557	12%	1.2%	VL	Short-term on-the-job training
Customer service representatives	2,202	545	25%	2.2%	L	Moderate-term on-the-job training
Combined food preparation and serving workers, including fast food	2,503	452	18%	1.7%	VL	Short-term on-the-job training
Office clerks, general	3,200	404	13%	1.2%	L	Short-term on-the-job training
Personal and home care aides	767	389	51%	4.2%	VL	Short-term on-the-job training
Home health aides	787	384	49%	4.1%	VL	Short-term on-the-job training
Janitors and cleaners, except maids and housekeeping cleaners	2,387	345	14%	1.4%	VL	Short-term on-the-job training
Nursing aides, orderlies, and attendants	1,447	264	18%	1.7%	L	Postsecondary vocational award
Bookkeeping, accounting, and auditing clerks	2,114	263	12%	1.2%	L	Moderate-term on-the-job training
Waiters and waitresses	2,361	254	11%	1.0%	VL	Short-term on-the-job training
Child care workers	1,388	248	18%	1.7%	VL	Short-term on-the-job training
Landscaping and groundskeeping workers	1,220	221	18%	1.7%	L	Short-term on-the-job training
Receptionists and information clerks	1,173	202	17%	1.6%	L	Short-term on-the-job training
Maids and housekeeping cleaners	1,470	186	13%	1.2%	VL	Short-term on-the-job training
Security guards	1,040	176	17%	1.6%	L	Short-term on-the-job training
Medical assistants	417	148	35%	3.1%	L	Moderate-term on-the-job training
Food preparation workers	902	138	15%	1.4%	VL	Short-term on-the-job training
Teacher assistants	1,312	137	10%	1.0%	VL	Short-term on-the-job training
Dental Assistant	280	59	22%	1.9%	L	Short-term on-the-job training

Source: November 2007 Monthly Labor Review Table 3 Bureau of Labor Stataistics November 2007 Monthly Labor Review. at www.bls.gov/emp
and Occupational Information at O*NET OnLINE!

New = Newly created jobs (not job openings which is new + vacancies)
O*NET gives "Projected need" which is new jobs + replacement for workers who leave.

Average yearly job growth rate = 0.8%

Salary
VH = very high $46,360 or more		
 H = high      $30,630 to $46,300	
 L = low       $21,260 to $30,560	
VL = very low  up to $21,220	

Books:
Now What?: The Young Person's Guide to Choosing the Perfect Career by Nicholas Lore

Industries with most job need (20% attrition assumed)
Job need = creation + replacement
Note: The need is based on an industry average attricion rate, which in reality is not consistent across industries.

Health and Medical Science:
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