It depends upon whether the UK remains within the European Economic Area or not. (Countries like Norway are outside of the EU but inside the EEA, meaning that they can still trade freely with the rest of Europe).
If the UK ends up outside the EEA many firms will switch all, or part, of their UK operations to countries within the EU. HSBC has already said that 1000 London-based jobs will be moved to Paris and several of the big American banks have indicated that they'll move their London offices as well. Further, it's almost certain that the Japanese car plants based in the UK will be closed down, and their operations moved elsewhere. (They only came to the UK to gain free access to European markets). Also, other foreign investors (such as major Chinese firms) that currently pump money into British businesses will cease doing so.
So, if the UK ends up outside the EEA, it's almost certain that an additional 2 million people will be made unemployed, with some analysts suggesting that the figure could well be over double that.
However Boris Johnson (and others who were associated with the 'Leave' campaign) are working on the assumption that the UK will end up in a position similar to that of Norway. (i.e. inside the EEA but outside the EEA).