I am advised by Revenue that the number of taxpayers who received the One Parent Family Tax Credit in 2013 and the Single Person Child Carer Credit for each of the years 2014 to 2018 (the latest available year) are set out in Revenue’s Cost of Tax Expenditures Publication, which are published on the Revenue website. (Search "Tax Expenditures" and select result "Costs of Tax expenditures (credits, allowances and reliefs)")
Tax returns for 2019 were due to be filed in late 2020 and the data from these are currently being analysed. Once this analysis is completed, the information will be updated at the above link. Information for 2020 and 2021 will not be available until tax returns for those years are filed and processed.