Complex sentence: The group itself estimates that perhaps 2.000 Greeks practice in the ethnic Hellenic tradition and another 100.000 have "some sort of interest", although this has not been independently verified and the group admits that the numbers of adherents are difficult to estimate.
Split into simpler sentences: The group itself estimates that perhaps 2.000 Greeks practice in the ethnic Hellenic tradition and another 100.000 have "some sort of interest". Though this has not been independently verified and the group admits that the numbers of adherents are difficult to estimate.