As of February 2015, 113,960 working-age claimants of Department for Work and Pensions benefits were EU migrants (that is, when they first registered for a National Insurance Number, they were a national of another member state). This represents 2.2% of total claimants.
Most of these were claiming an “out of work benefit” such as Jobseekers Allowance. However, the total includes some benefits, such as Housing Benefit and Disability Living Allowance, which can be paid to those in or out of work.
As of March 2014 317,800 families who were in receipt of tax credits, 6.8% of total claimants, contained at least one adult who was an EU national at NINo registration.
For comparison, those born in the EU (outside the UK) make up about 6% of the working age population. Statistics are not readily available for pensioner benefits, which account for a large proportion of overall benefit spending, but given the age profile of EU migrants they are likely to make up a very small proportion of both the number of recipients and of expenditure.
Related questions include Would leaving the EU reduce immigration to the UK? and What would UK immigration policy look like after Brexit?
Further Reading: try this piece by Matt Bevington on immigration post-BrexitView all facts