What is the highest possible thrust generated by 15cm ducted fan?
A 15 cm diameter fan could maybe lift 4 kg of mass with a tip speed of 0.6M.
By increasing the power and the blade chord length (in other words, increasing the solidity of the rotor), this may be increased to maybe 6 kg or more, but a single fan would probably never lift more than 10 kg mass.
The thrust of a rotor is $C_T \cdot \rho \cdot \text{Area} \cdot \text{tip speed}^2$.

$\rho$ = air density ($1.225~\mathrm{kg~m^{-3}}$ at sea level).
$\text{Area}$ = area of rotor disc ($\mathrm{m^2}$)
$\text{tip speed}$ = the linear speed at the tip of the rotor = radial speed * radius.
$C_T$ = coefficient of thrust. (is usually in the order of 0.02 to 0.05 for small rotors)

Assuming $C_T$ of $0.03$, the calculation shows $40~\mathrm{N}$ ($=4~\mathrm{kg}$) thrust.
Disc area is one of the most important parameters, even a 30 cm disc would generate 4 times the thrust. 60 cm would make almost 65 kg.
Apparently, a 50 cm diameter disc can possibly generate 45 kg thrust on its own. So two of them could carry and maybe even lift-off a person equipped with this backpack (the person + the system must be less than 90 kg).