There are five different alkenes that result when 3-methyl-2-pentanol is heated with concentrated H2SO4. This is because the reaction proceeds through four possible carbocation intermediates, each of which can form a different alkene.

1. Formation of carbocation intermediates
When 3-methyl-2-pentanol is heated with concentrated H2SO4, the acid protonates the alcohol group, forming a carbocation intermediate. There are four possible carbocation intermediates that can be formed, each with a different position of the positive charge on the carbon chain.

2. Rearrangements
Once the carbocation intermediate is formed, it can undergo rearrangement to form a more stable carbocation. This can occur through hydride or alkyl shifts, which move a hydrogen or alkyl group from one carbon to another. These rearrangements can result in different carbocation intermediates than the ones formed initially.

3. Formation of alkenes
Once the most stable carbocation intermediate is formed, it can react with a neighboring hydrogen on an adjacent carbon to form an alkene. Since there are four possible carbocation intermediates, each of which can form a different alkene, there are a total of five different alkenes that can be formed.

4. Stereochemistry
It is worth noting that the reaction can also result in the formation of stereoisomers, depending on the position of the hydrogen and alkyl groups around the double bond. However, since the question does not specify whether or not stereochemistry is considered, we can assume that the five possible alkenes are constitutional isomers only.