As many as the police decide to give you. And therein lies their fault.
Cautions were originally introduced to give first time offenders who have admitted their guilt to a trivial offence the chance to avoid prosecution and conviction by accepting a caution. It was really intended for people who may have been subject to a Conditional Discharge or perhaps a small fine if the matter went to court. Such has their abuse developed that they are now offered to serial offenders and those committing serious offences.
The government has introduced new guidelines which should see a cut in the number of cautions issued and a trial is currently in place:
http://www.bbc.co.uk/news/uk-29859758
But the fact is this well-meaning idea has been blatantly abused by the police to save them the trouble of pursuing a prosecution and it needs to be more rigorously controlled.