Easiest way to do that is use CBSubs Family Plans. The owner of the subscriptions then has full management rights over who gets that subscription. They'd just share it with their employee accounts.

The number of times a plan can be shared can be a static value or based off a user field. In your case probably user field is best so you can adjust the field based off the number of licenses they purchase. As for purchasing additional licenses you've a few ways to do that, but the easiest is probably CBSubs Options as it can provide a field for them to select additional licenses.

Coming back on this subject, how can I set up a dropdown box or an input box in Options so that a user could buy 10 licences on a single purchase ?
I already use Options Prices to give them the choice between monthly or annually plan subscription but cannot see where to add the info to buy several subscriptions at once.

You can add plan options under Integrations > Options > Options. These are basically extra price fields for plans. There's several fieldtypes available there including dropdown select.

Also for the Family plan shared subscription option, does it mean that a single subscription can be shared say 5 times or does it mean that a single user has to buy 5 subscriptions to be able to share it 5 times, and how to achieve it ?

It limits the number of times they can share that plan. They do not need to buy it 5 times to share it 5 times. If set to 5 after buying the plan they can share it to 5 active users at any given moment.