I have found the following sentence in my practising book (only 日本 is written in kanji because it's in one of the first lessons):
日本は しまが 6852 とう あります。
You are correct on both accounts. With the kanji for 島 being used, this sentence would be written as
日本は島が6852島あります。
The first 島 would use the kun reading しま and the second would use the on reading とう.
Source: http://www.benricho.org/kazu/shi.html