How determine the number of LEDs that I need?
Luminous flux (lm) is not what we want- it incorporates the sensitivity of the human eye. 

For UV curing we need irradiance in W/m^2 and an exposure time to get a total integrated energy that is sufficient. We also need to consider the peak wavelength of the UV LED and the sensitivity of the material you are polymerizing to that particular part of the spectrum. Different materials are more or less suitable for a given wavelength, and LED light sources that provide shorter wavelengths tend to be more expensive and perhaps shorter life. 

You can always increase the exposure time to increase the total dose, but be careful about trusting the claimed output power of 'no name' light sources, and be careful about running them at the full rated current. It would be best to derate the latter by at least 30% and to keep them cool via a heatsink and fan or closed-loop liquid cooling). 

It may also be somewhat difficult to estimate the actual exposure with an adhesive or thick material (or if the material is partly blocked by something like glass which absorbs some of the UV) vs. a simple overcoat or UV ink cure, so you may have to experiment.