As more or less in real life, injuries are just bad luck, so there is no limit how often you have to deal with injuries in one season.
Normally it should be around 1-3 injuries per season, but you can have one or more seasons without any injury and then there will be times where you get 3-5 in one season.
Injuries can not end the players career, but e.g. you have a player with 22 years of age he recovers faster, than a player with 36 years of age. It means younger players will have more short injuries like 2 or 3 days and older more injuries with 4-5 days.
As we play a relatively short season with just 22 gamedays in the regular season, the longest possible injury is 5 days.
You can not dress up a winger as defenceman or center, at least not yet. So you should have a bench player for each position, but most managers use just amateurs for that, so save money. If you login daily you can add backup players just in time when you need them, too. In case you do not login daily, i recommend that you have bench players for each position, they will be added to your lineup automatically if needed.