2 art + rest inf for 20 width. supports: art, signal, engineer, hospital, aa
I wouldn't do this anymore. A lot of guides say 7-2s are the meta, but they're all outdated. Ever since the artillery soft attack nerf 7-2s have been inefficient and expensive. 20-width infantry is bad offensively in general, so spending artillery on them is a waste of resources. If you're making 20-width infantry, the only role they will be good in is a defensive one. The best defensive infantry division is one that is pure infantry, since infantry has the highest defense stat of all battalions. It will have lower soft attack, but the goal of a defensive division isn't to inflict casualties but to hold the line and not run out of organization. Adding artillery to an infantry division both lowers the defense and the organization of the division, so it is counterproductive to these goals. Also, unless you are running superior firepower, you should not put support companies other than engineers in your infantry as they lower the organization too much. Engineers increase the entrenchment of your infantry, so they're a must for the defensive role, along with providing terrain bonuses to the division. Other support companies are not useful for such a purely defensive division and are a waste of resources. There is one exception - if you want to make your 20-width infantry more beefy and have superior firepower integrated support, putting support artillery and support rocket artillery in a 20-width infantry division is very efficient in terms of IC cost per soft attack(but also more expensive).
EDIT: as
@squid_hills said, support anti-air is also a good thing to include if you expect to face enemy air superiority, as it can blunt the effects of the air superiority penalty as well as bleed the enemy CAS. If you have friendly air cover it's largely a waste though.
END EDIT
If you want infantry to perform an offensive role, your divisions should be 40-width. 14-4 works, as a 40-width is big enough to have the defense stat to perform well on defense against the AI, and a soft attack stat high enough to make successful assaults against the AI. Keep in mind, a 14-4 will not be able to push into tiles defended by 14-4 divisions without overwhelming numbers and air superiority, because infantry just naturally has much higher defense than it has attack. Two equal infantry forces battling in a vacuum will result in the defender winning 100% of the time just because of how the stats work.