Stellaris\common\deposits
...is responsible for all the resources that can spawn.
01_orbital_deposits.txt for all the resources that can spawn in the galaxy, determined by a weighted, random roll.
00_null_deposit.txt has the null_deposit, which is the random chance that no resource will spawn. Lowering its weight is an easy (but inaccurate) way to increase the amount of resources in the galaxy.
Keep in mind that the null_deposit affects ALL deposits though, including those found on habitable planets (those are the things found in the "Features" Tab of the planet UI, including blockers). So if you make changes to the weight of the null deposit, make sure you exclude habitable planets, or they'll have increased amount of district slots available.