There are 15 sigma bonds in the molecule. These bonds are formed by overlap of atomic orbitals.
There are two kinds of bonds in organic chemistry, sigma bonds and pi bonds. Sigma bonds are formed by end to end or head to head overlap of atomic orbitals.
Pi bonds are formed by side by side overlap of atomic orbitals.
In the compound 4-methyl-pent-2-ene, there are 15 sigma bonds and only one pi bond.
Hence, in the compound 4-methyl-pent-2-ene, there are 15 sigma bonds.
For another definition of sigma and pi bonds see:
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