In Zhejiang Jiaxing, if the social security base is 19,900 yuan, the amount in the medical insurance card each month is calculated based on specific policies. Generally, for employees, the amount in the medical insurance card is related to factors such as personal and unit contributions, and age.

1. Employee contribution part:

• Employees need to pay 2% of the social security base for medical insurance. If the social security base is 19,900 yuan, the monthly personal contribution for medical insurance is ​
  ×2%=
  yuan. This part of the money is directly deducted from the employee's salary and is an important part of the amount that may be credited to the medical insurance card.

2. Unit contribution part and its allocation related to the medical insurance card:

• The unit's contribution rate for medical insurance is 9% (including 0.5% for employee maternity insurance). So the unit's monthly medical insurance contribution is ​
  ×9%=
  yuan.

• A part of the unit's contribution will be allocated to the employee's personal medical insurance account. The specific allocation ratio is related to the employee's age.

• For employees 45 years old and below, the amount allocated from the unit's contribution to the personal account is calculated in a certain proportion. Generally, in addition to the personal contribution, the unit - allocated part to the personal account for employees 45 years old and below may make the total amount in the medical insurance card reach about 150 yuan per month (the sum of personal and unit - allocated parts related to the personal account). Considering the personal contribution of 398 yuan, the total amount credited to the account from both personal and relevant unit - allocated parts may be adjusted around this value according to the actual situation.

• For employees over 45 years old, the amount allocated from the unit's contribution to the personal account is different. Usually, the total amount in the medical insurance card (including personal contribution and relevant unit - allocated parts) may be about 160 yuan per month. Given the personal contribution of 398 yuan, the overall amount credited to the account will be adjusted based on the actual unit - allocation policy.

3. For retirees:

• If it is a retiree, the amount in the medical insurance card is calculated differently. Retirees generally do not need to contribute the 2% personal medical insurance contribution. The amount in the medical insurance card for retirees in Jiaxing is usually around 195 yuan per month.

In conclusion, for employees in Jiaxing with a social security base of 19,900 yuan, the amount in the medical insurance card each month is affected by personal contributions, unit contributions, and age. The specific amount can be accurately determined by consulting the local social security department or relevant insurance agencies.

进一步了解嘉兴社保政策的其他方面，比如不同险种的报销细则，或者社保基数调整的影响，都可以随时问我，我很乐意为你提供更多信息。