For counting bacteria, an oil immersion lens is usually required. Many of these are computed to result in an objective-to-specimen distance of about 0.25mm. Considering that a standard #1 cover slip is ~0.17mm in thickness, the remaining minimal gap can be bridged by immersion oil.
Covers for counting chambers are polished to be perfectly flat, and since capillary action exerts suction, a very thin slip could result in an inaccurate count. ProSciTech supplies Helber with thicker than standard slips but they are suited to work with standard oil immersion lenses.