How many people does the Greater London Authority Group Employ

The GLA employs around 620 staff. The cost of administering the GLA in
2004-05 will be £73m. Most of this is met by central government grant.
London council taxpayers contribute a small amount ? about 14p a week
on a Band D council tax bill.

The GLA is made up of the directly elected Mayor of London and a
separately elected London Assembly.
There is a clear separation of powers between the Mayor - who has an
executive role, setting an overall vision for London and defining
clear strategies - and the Assembly, which has a scrutiny role.

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source:http://www.london.gov.uk/london-life/city-government/gla-group/index.jsp

Also note:
The Mayor has an office of around 30 staff who support him in his role
and direct staff on his behalf.