Assistant coaches help with training. A team may only have 10 of any one type of staff. However assistant coaches and goalkeeping coaches are treated as one type, so a team may only have a combined total of 10 coaches. Typically non-goalkeeping training teams employ 9 assistant coaches and 1 goalkeeping coach. It is believed that a goalkeeping coach helps to maintain the form of the keeper. Other people prefer a 8 to 2 split between coaches.
Some users claim it is possible to have 11 coaches without seeing any ill effects, but many users have lost training when trying this.
It is not always in a teams best interest to hire all the possible assistants. The effect of each additional assistant decreases fast. E.g. having 5 GK coaches instead of 10 will slow GK training all the way from solid to divine by 1 week only. This is approximately 2 years of real time!
Clubs in financial difficulty should always bear this in mind when trying to make savings.