While I won't say you can put as many as you want, inverts general contribute little to the bio-load, and are beneficial in breaking large waste/excess food down. Fish contribute more waste to the system than an equal amount of inverts (many snails, hermits, and shrimps are detrivores, most fish are not).
Just keep in mind that like vertebrate critters, inverts have food, and space needs that must be addressed. For example, coral banded shrimp tend to be agressive towards other shrimp. Arrowhead crabs are territorial, and will frequently kill intruding shrimp and crabs. Some kinds of hermits are viscious shell stealers, and will kill snails just to try a shell on for size. Some snails are herbivores, and if the algae supply is limited, will die in droves and pollute the tank. Similarly, some species will thrive, and their reproductive pattern can be a bit polluting--they spew eggs and sperm out in clouds.