SOT-223 Thermal Pad and Vias
Can you post the part?

The best advice I can offer is first to understand that LDO dissipation is simply related to (Vin-Vout) * Iout, where Vin is the input voltage, Vout is the output voltage and Iout is the output current. In general, you shouldn't need to worry too much for less than 500mA. However, you can take the datasheet of the LDO and look at the thermal resistance. This tells you exactly what will be the temperature rise given a certain wattage dissipation.  You can calculate the dissipation with the formula above. For example LM7805 has 5 degrees per watt. So, you can expect a rise of 5 degrees for every watt you calculate it dissipates.

So, what I would do if I were you is:

1) Calculate the expected wattage dissipation. Be conservative.

2) Calculate the expected temperature rise. Anything 40-50 degrees is likely not an issue at all. Close to 90 degrees and over you almost have to take care of it.

3) Pick the right package. Different parts have different packages, each with different thermal resistance. Usually larger packages and those with tabs for power dissipation have lower resistance, hence they will heat less. If the temperature is not acceptable,you might need to pick another part.

Look at the following video from Dave at EEVblog. Dave does an awesome job at explaining all the terms and how to do the design. Should cover all you need.

Specifically to address your vias, clearly more vias enable more heat to be sinked to other plans and hence increase the heatsink, but it's important to know how much area you need. Datasheets often recommend the area in mm^2 required for certain thermal resistance.