The amount of roving you need to buy will depend of several things. To give you examples of considerations for calculations I have chosen the Merino wool I supply at www.SarasTextureCrafts.com
Length – The length of your dreads. 100g of my coloured Merino is approx. 410cm/4.10m in length and as I hold my stock in mostly uncut lengths if you were to order 2 units of 100g they would come as 1 very long length (approx. 820cm, or 8.2m). So you can cut them to your length as you work. You will find some colours may measure longer and other shorter, it truly depends on thickness and dyed weight of the fibre as they come off of the mill machine.
Width – The width of your dreads. The width of roving varies per colour and is slightly uncontrollable along the length so it is something that we don't give dimensions on usually as fibre sellers, because if you come back it may vary by a small degree.
Number and head coverage – How many dreads do you need to make.
Whilst I will not be able to calculate your project for you, I can tell you what most of my dread makers buy and how they buy it;
For pencil thin dread fall attachment dreads they buy 200-300 grams per colour and probably get around 4-5 dreads per 1m length. Rough calculation would estimate that this could give you around 12-15 pencil dreads depending on how you work.
For pencil thin dreads that are permanent full head attachments, they usually buy between 500-600g per colour.
As I'm not a dread maker myself, I can't always accurately advise what is best as design, colour, head cover and length all play apart in working out how much you might need... but I hope the above information has given you some idea?