While it may seem that a two-ply bed sheet would be a better choice with the greater number of fibers it contains, this isn’t usually the case. Two-ply sheets are often rougher and less durable, as cheaper materials are typically used. Focus more of your efforts on finding the right material and weave, but do try to stick with one-ply sheets.
A bed sheet is a rectangular piece of cloth used either singly or in a pair as bedding, which is larger in length and width than a mattress, and which is placed immediately above a mattress or bed, but below blankets and other bedding (such as comforters and bedspreads). A bottom sheet is laid above the mattress, and may be either a flat sheet or a fitted sheet (a sheet which has been sewn with elastic or a drawstring along the hem of the top, sides and bottom of the sheet to prevent it from slipping off of the mattress). A top sheet, in the many countries where they are used, is a flat sheet, which is placed above a bottom sheet and below other bedding.