Hello, in our opinion we don't believe this to be much of an issue, with the information you have presented the vast majority of real life teams stick to having just five foreign players.
Foreign players tend to be first teamers or star players and it makes little sense given the foreign player limits in Japan for the AI to plan squads with extra foreign players when they would not be able to name all of them in the same matchday squad if all were fit.
It may be in real life that some teams have an extra foreign player or two in the squad in case of injuries, or with some kind of pre-agreed rotation system in place, or even have backup foreign players, but it would take a lot of complicated gameplay coding changes to get this to work sensibly in FM.
Obviously if you're the human manager you sign more foreign players if you wish. But ultimately we don't believe this to be much of an issue.