You should send a PM to Radioman as he will know for sure.
Usually you will want pairs as each pair is designed to be an equal portion of the wavelength needed for AM reception. For example if you needed a 9 foot antenna for AM, you would use 2 x 4.5 footers connected together. You can google AM antenna lengths and see how your lengths compare.
Whether it's best to have the pair on one side or one on each side I don't know. You might be able to make one more element using something like steel banding used for pallets and cutting your own insulators from rubber mud flaps.
AM is very easy to receive so I suspect almost any combination will work for local stations but if you are trying for distant stations or correct restoration, then you might need pairs.
1938 Canadian Pontiac Business Coupe (aka a 1938 Chevy Coupe with Pontiac shaped front sheet metal - almost all Chevy!) 1975 4-speed L82 Vette