The number of slides that may be coated is dependent upon the surface area. The general use instructions recommend using 0.5 - 1.0 mL per 25 cm2 coating area - the 25 cm2 value is based on a T-25 tissue culture flask. A standard microscope slide has an approximate area of 1.3 cm2.
Please see the link below to review the product datasheet which included a general protocol for coating: https://www.sigmaaldrich.com/deepweb/assets/sigmaaldrich/product/documents/784/704/p6407pis.pdf