Use singular or plural for the noun after 's.

Hi guys, I have a grammar question which has been bothering me for a long time, please let me know if this is inappropriate in this forum.

I cannot distinguish whether I should use a singular noun after 's, or a plural noun. For example:

The countries' ambassadors - I know it means 'the ambassadors of more than one country', but can it tell that one country has many ambassadors, or it means one country has one ambassador so many countries in total have many ambassadors?

2nd example:
James and Julie are a couple.
Their bank account - does it mean they share the same account, or it means each of them has only one bank account?
Their bank accounts - does it mean each of them has more than one bank account, or it means one member has one account, so a total of two members has two accounts?

3rd example:
James has James Pension 1 account and James Pension 2 account
Julie has Julie Pension 1 and Julie Pension 2.
If I want to refer to James Pension 1 and Julie Pension 1, should I use their respective pension 1 account/accounts?

Thank you very much.