How many Solar Panels for a 100w Compressor?

That rather depends on how big the panels are.

The local climate also makes a huge difference.  As does the latitude.

You need 2.4 kWh per day. On a sunny day, a 1.5 kW panel will just provide what you need because you need to allow for charge discharge losses. On a cloudy day you will probably need 5kW. In winter you will need more. My guess is 10 kW plus about 20 kWh of storage.

Yeah, what KiwiBuzz said. For UK allow for 10x the power as a starting point 100W x 10 = 1kW, as that will do it for cloudy days. Then factor in winter daylight is like 40% of the time, so divide by 0.4 gives 2500W, then double it for charging losses, really long spells of carp weather, and you get 5kW. Typical large panels these days are 400W so you'll need 12 or so. Roughly. Probably. For storage you want lithium as lead-acid you can only run to 30 or 40% discharge, and you want enough storage to cover long spells of winter cloudy weather so enough to run for 4 days continuous which is 100W x 24 x 4 / 80% = 12kWh. Probably.

Primarily , take into account the solar radiance available  at the proposed location . then the conversion factor , that is efficiency of the panel . For instance, if  the location has a solar irradiance of 600 watt,  and  the proposed solar panel is 18 % efficient in conversion, theoretically, you will get  108 watt per solar panel of 1 sq.m.  You need 2400 Watt per day assuming the wattage requirement is 100 W. Consider energy losses in calculations. if the sun is available for 4 hours only, you need a storage for the remaining 20 hours, that is 2000 watt storage . ideally you need minimum 6 sq.m solar panel of efficiency 18%,   to power your compressor  for the full day. This is assuming , sun availability of 4 hours and the other conditions stated above.