There are lots. Start by looking at the chain as 6 carbons long and number them. You can put the double bond either between carbons 1+2 or 2+3 or 3+4. 4+5 would be the same as 2+3 so you stop here. The first three isomers. Now make a 5 carbon chain with a double bond between carbons 1+2. You can put the extra carbon (methyl group) on carbons 2,3 or 4 making another three isomes. Now move the double bond to between carbons 2+3. Again another three isomers. You need to continue to eliminate all options-take care that you don't repeat the same isomer! Remember that carbon can make 4 bonds so if it is involved in a double bond there are only 2 other bonds it can make.
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