How many people in the United Kingdom have the title "Lord"?

In the UK there are 666 Lords (this is the value as of March 1st 2004).
http://www.parliament.uk/works/lords.cfm

Being a Lord (or Lady) in the UK used to entitle you to sit in the
House of Lords although this is all changing.  The House of Lords is
the upper chamber in UK parliament and is not (as yet) democratically
elected.

There is a very imformative FAQ here:
http://www.parliament.uk/faq/faq2.cfm

And the UK Parliament site in general is a good starting point for
further research on the title "Lord":
http://www.parliament.uk

If you have any further questions or queries regarding this matter
please ask and I will be happy to provide you with an answer.

Aha!

666 ... The Number of the Beast ...

How significant!

Not necessarily correct. The Younger sons of Dukes, Marquesses and
Earls are addressed by a courtesy title of Lord. As are all the
hereditary peers who no longer sit in the house of lords.

666 is just the number sitting in the lords (the vast majority being
life peers - ie their heirs will not inherit the title) There are many
more hereditary honorific titles as already mentioned and more life
peers are created twice a year and on occaisions such as the stepping
down of a Prime Minister (remember Harold Wilson's lavender list?)
Plus one can buy some really crappy and useless Lord of the Manor
titles from various dealers (they sometimes come with a house -
they're not 'real' lordships really)