Latest estimates of the number of tax credit recipients, including those families receiving working tax credits, are available in Table 1.1 of the HMRC provisional awards snapshot publication, "Child and Working Tax Credits Statistics" as at December 2008. Table 2.4 also gives the number of families benefiting from the child care element of working tax credit. The publication is available on the HMRC website at:
http://www.hmrc.gov.uk/stats/personal-tax-credits/cwtc-dec08.pdf