Do you have any previous information on the species you're working on? You'll would need to have an idea on the extent of LD in the species, so that you can estimate the number of markers taking that into account, together with the estimated genome size.
Thank you for your reply. in the population which I use, The LD decayed at r2 = 0.16 at 10.36 cM. So, you mean that I have to take all marker pairs which showed LD (r2) less than 0.16