How many trophic levels can an ecosystem contain?

However, theoretically there could be many more. It is only limited by the number of food chain niches that are possible. General classification by food source results in the four main trophic levels.

It is possible to have quaternary and higher “consumers”, as even secondary consumers may also use the first trophic level as primary consumers.
http://www.ck12.org/biology/Trophic-Levels/lesson/Trophic-Levels-BIO/

Trophic Level Where It Gets Food Example
1st Trophic Level: Producer Makes its own food Plants make food
2nd Trophic Level: Primary Consumer Consumes Producers Mice eat plant seeds
3rd Trophic Level: Secondary Consumer Consumes primary consumers Snakes eat mice
4th Trophic Level: Tertiary Consumer Consumes secondary consumers Hawks eat snakes

Generally only about 10 percent of the energy at one level is available to the next level. This is represented by the ecological pyramid in Figure below.

What happens to the other 90 percent of energy? It is used for metabolic processes or given off to the environment as heat. This loss of energy explains why there are rarely more than four trophic levels in a food chain or web. Sometimes there may be a fifth trophic level, but usually there’s not enough energy left to support any additional levels.