Interesting question! I can't really help because I don't know how many cells there are, but I would suggest do some estimations for example:
If you know the mean size of endotelial cells (therefore the volume assuming a romboid morphology) and the mass of the endotelial heart tissue you can divide this total mass of this tissue by the mass of a single cell (assuming a 1mg/mL density of the cells for instance).
Other option can be stimation by manual cell counting of x uL of cell suspension (in a Neubauer chamber, with or without Trypan Blue staining and other counting techniques).