French infantry battalions still used the six-company organisation in 1815, i.e four companies of fusiliers, one of grenadiers and one of voltigeurs. Since you are planning 24-figure battalions, I'd advise stands of four, each four-figure base representing an individual company. This also allows you to accurately depict actual tactical formations on the table - line, column, column of divisions and square. Light and line regiments used the exact same organisation, the only difference being that in light units, fusiliers were called chasseurs and grenadiers carabiniers.
How many command figures you decide to include in your units is up to you : most people will agree that three (officer, eagle or fanion bearer and drummer) is the accepted minimum. If you are so inclined, you may want to add a sapper along with one or two NCO's but that is entirely optional : wargames units are by definition a stylised representation of reality, so I wouldn't get too worked up about what's 'real' and what's not. In the end, what's 'right' is chiefly what's most aesthetically pleasing to you, provided the six-company structure of a battalion is correctly rendered on the table.
As Grand said, bear in mind that only the first battalion in each regiment carried an eagle, so pack FN 5 should be reserved for you first battalion. The other battalions had square fanions rather than eagles and since FN 6 was obviously designed to provide command for two battalions, a single pack will provide enough figures for your second and third btns.
In terms of what you need to purchase for a three battalion regiment, here's what I would recommend :
- 1 x FN 5 (command for the 1st battalion)
- 1 x FN 6 (command for the 2nd and 3rd battalions)
- 6 x FN 9 (36 fusilier figures)
- 4 x FN 11 (24 flank company figures, to be divided into three grenadier companies of four figures each and three similar companies of voltigeurs)
This gives you a neat total of 72 figures to be divided into three 24 figure battalions, with no wastage or extra miniatures. If you are so inclined, you may want to add one FN 13 and one FN 16 pack to your order to add NCO's and casualties to your units but that's entirely a matter of personal preference. For light infantry battalions, substitute code FN 71 to 74 to the ones mentioned above, or FN 76 if you prefer the later lentil-shaped pompons for your chasseurs.