OK, so I found something which more or less answers the question. Presentation Aerosols and Clouds on Earth and Venus from Owen Brian Toon of Department of Atmospheric and Oceanic Sciences and the abstract of BEYOND SULPHURIC ACID – WHAT ELSE IS IN THE CLOUDS OF VENUS?
In short the density of the clouds is about 1-50 $mg / m^3$ of liquid phase. Clouds are formed from 75-90% $H_2SO_4$. 6e+13 kg of $H_2SO_4$ is produced each year by photochemical processes.
But still it does not answer exactly what is the total amount.
