This is stated in a response to a request to the IDF, published by the British publication Declassified.
According to the document, the data is current until March 2025. It covers military personnel with dual citizenship (Israel and one other country), as well as with multiple citizenships (Israel and several other countries).
The report notes that some individuals may be counted in both categories simultaneously. This applies to cases where an individual holds one specific foreign citizenship, which is counted as dual citizenship, as well as additional citizenships, which are classified as multiple citizenships.
According to these data, Ukraine ranks fifth in the number of its citizens serving in the IDF. The largest number comes from the United States (12,135), France (6,127), Russia (5,067), and Germany (3,901).