In my opinion, 8 to get full coverage for flowering, 4 for full coverage for vegetation. This is based on 4 sq.ft. per light for flowering and 9 sq.ft. per light for vegetation. The 8 HGL-100s will run ~$1200.00.
The HGL-600H is ~$760.00 and is designed to cover 16 to 25 sq.ft. This would seem to be the better way to go... \