Steve's answer is correct but I suspect that what you really want to know is the quantity of 'duty paid' cigarettes you can bring back.
When travelling from any EU country (with the exception of some of the 'new entrant' countries, in eastern Europe) you're officially entitled to bring back an unlimited quantity of cigarettes as long as they're for your personal use. (The definition of 'personal use' includes genuine gifts for other people, for which you receive nothing in return, but excludes cigarettes for which you'll receive some form of payment or exchange).
HMRC have issued guidelines for the amount of tobacco and alcohol which people can bring back, for their personal use, before they'll normally start asking questions. The relevant figure here is 3200 cigarettes. It should be noted, however, that you might still be questioned by HMRC (and risk having your cigarettes confiscated) if there are circumstances which suggest that the cigarettes you're importing aren't genuinely for your own use. (e.g. if there are several different brands).
If you find yourself tempted to go 'a bit over the guideline', you should be aware that you risk confiscation of all of the cigarettes you bring back, not just the excess ones.
Chris