Hello
depending on what applications and functionalities your 15 users will need (sales enterprise or professional, customer service enterprise or pro, field service) you will need the corresponding licenses.
Licenses price can be checked here are bellow https://dynamics.microsoft.com/en-us/pricing/
Note, this sales motion is user license (USL) based. Only the Business Applications that are USLs may be purchased as an Attach license. 1. Purchase the first or Base license at standard pricing 2. Purchase additional eligible Attach licenses at a reduced price

you can check all the details on the D365 Licensing guide
Queries.
1. Will Microsoft give dev , test instances along with main production instance,
To create an environment with a database, you need 1GB available database capacity as MS has changed last year to a Capacity model and all the instances addon licenses were removed from price list.
The first subscription (Base license) of a Dynamics 365 workload application includes the default capacity which is shared per tenant. Additional subscriptions do not include additional default capacity. Note, Attach licenses are licensed to access the platform entitlements included with the Base application. For example, if a new customer purchases Sales, they will receive 10GB of default Common Data Services Database Capacity. If an existing Sales customer purchases Field Services, they will not receive additional Common Data Services Database Capacity above the initial 10GB.
2. will there be any offerrings for different service user and test user and dev users?
all users are managed trough Office365 and can access different organizations depending on the security model you will implement, like organization security groups.
if you are referring to lighter users check the info on licensing guide to understand the Users rights with more detail. (Appendix B: Customer Engagement Applications Use
Rights)

Teresa