It supports the deductive inference from your evidence to your claim. A research hypothesis, guiding your investigation, generally draws upon the accepted explanatory system to make an inference from new observation. The explanatory system and the rules of logic provide the warrants for these steps, bridging the evidence to the claim. You also have to get the source's context right, with no sins of omission or misreading. However, a strong piece of contrary evidence against a broad claim can be enough to dismiss it.
Quality can overwhelm quantity.
For primary sources, you need to look for complete, well-documented editions. The internet, for example, has a lot of textual material, but editing standards are non-existent and you're at the mercy of the site's accuracy or care. Show us the meaning. Challenging yourself and your favorite notions is the mark of the strong intellect. Ask yourself how much data or how many different kinds of data you really need before making a commitment to a large belief.