It's possible to calculate the losses for a given configuration, but it's not an easy task. Meanwhile, there is a hundred years worth of testing and experience with propellers, so if you have a classic prop design and want to know typical trends, you can find most of the results in the literature.
One of the best classic sources on this topic is NACA-TR-640 (1938!)
I'll quote one graph from it that is directly relevant to the question:
As can be seen, adding an otherwise identical blade causes a power loss of maximum 6-8% per blade (in static conditions, at zero speed), and very little (1-4%) in normal flight conditions.
Comparison of the respective propulsive efficiency curves (Fig.35 in the work) is even more interesting: the difference is even smaller, and in some regions (when the blades stall) the multi-blade props can be more efficient than two-blade ones.
In practice, when you need to absorb extra power from the engine and are constrained by the diameter, you have to increase solidity of the prop. You can do it either by adding blades or by increasing the blade chord. The paper demonstrates that adding blades is better in terms of efficiency. Thus the premise "The ideal propeller has one blade" is not necessarily true, given the realistic constraints.