Predicting the number of addition products
Addition reactions of alkenes with symmetrical reagents such as Br2 and H2 with alkenes always give a single product.
Addition reactions of alkenes with asymmetric reagents (like A-B or HCl or HBr) give two products IF the alkene is also asymmetric. 
How do I recognize if an alkene is asymmetric?
An alkene as asymmetric if the two groups bonded to each of the doubly-bonded carbons are the different.
CH3CH2CH2CH=CHCH3 is asymmetric
CH3CH2CH=CHCH2CH3 is symmetrical
This is because the two halves of the reagent are the same.
Addition reactions of alkenes with asymmetric reagents (like A-B or HCl or HBr) give two products IF the alkene is also asymmetric.
How do I recognize if an alkene is asymmetric?
An alkene as asymmetric if the two groups bonded to each of the doubly-bonded carbons are the different.
CH3CH2CH2CH=CHCH3 is asymmetric
(H and CH3 on one doubly-bonded carbon; H and CH2CH2CH3 on the other)
CH3CH2CH=CHCH2CH3 is symmetrical
(H and CH2CH3 on both doubly-bonded carbons)