Thickness:
Batts usually come in either a 75mm or 90mm thickness. Match the thickness to your wall stud thickness.

R-value:
The R-value is a measure of resistance to heat flow through a given thickness of material. The higher the number, the higher the resistance and therefore the more effective it is.

Area per pack:
The number of batts you receive in a bag can differ by brand. Make sure you compare the area per pack for each product. The calculated area usually has an allowance for studs.