Density of cells in human tissues?
Well, Sender, Fuchs, and Milo wrote a paper to discuss the total number of cells in the body and compare it to the number of bacteria in the body (http://dx.doi.org/10.1101/036103). Their discussion is quite in-depth.

There are a number of ways to calculate it, but an interesting one involved calculating mammalian cell density by a study measuring the DNA obtained from a 25 g mouse. Researchers were able to determine there were 3*109 cells in the mouse (Baserga, 1985). From this study we can simply divide to find a value for "typical" mammalian cell density--1.2*108 cells/g.

If you want to know the number of cells per mL, you'll need the density (g/mL) of the tissue you are measuring. As the comments have mentioned, this is typically around 1 g/mL and varies from person to person. There is a web page that has some tissue densities for various tissues.

Baserga, R. (1985). The Biology of Cell Reproduction (Harvard University Press).