As has already been mentioned, you have a fair amount of flexibility when deciding which subscriptions and features you're gonna use in Microsoft 365. It's important to understand what your business and user requirements are in order to make sure that you've enabled the services that you need, without over provisioning and costing your business unnecessary money. The way you do this is by making sure you have a clear picture of what your needs are. 

Some things like email or Exchange are pretty straight forward and is probably the most used service in Microsoft 365. However, some of the less clear choices when configuring your tenant, might be around whether or not your users need to have Office Pro Plus installed. If you do need Office Pro Plus, you need to have E3 licenses. Do you need them for all your users or only some? Can some of your users do their job functions entirely on the web or do they all need to have the Office Client installed? Let's take Exchange Online for example, you might know for sure that you need to have Exchange Online as part of your base feature set. But do you know if you need some of the advanced features that come as part of the different license suites? If your users have very limited needs, such as small mailboxes, they mainly use Web Mail or mobile for email and Office, than you might only need licensing for front line workers or the F1 subscription.

If on the other hand, you need to take advantage of the larger mailbox sizes, like a 100 gigabyte quota, with unlimited archiving. Then you need to consider a E3 subscription. A special consideration would be some of the advanced compliance and security features like e-discovery and litigation hold. These features are only found in an E3 or E5 subscription. Your business might have a greater need for data protection and compliance, or potentially require a highly secured, redundant authentication path to the Microsoft 365 services. This is going to cause you to want to architect your solution way differently, from a company that really only needs to get users up and running with email and Office as soon as possible with no over-arching security or compliance concerns. Not to say that you're going to build a second option out in an insecure manner of course, but it will definitely change the details of what needs to be configured, and the steps you need to take before you start to actually migrate any users or data into Office 365. Your collaboration requirements are also going to have a fairly large effect on your planning. Does your company need only email to start? Or do you also need to start implementing Teams, Sharepoint and OneDrive right away? What about Microsoft business apps, such as, Bookings, Microsoft Invoicing and Staff Hub, do you need to start providing access to these features and services right away? 

It's a really good idea to spend some time familiarizing yourself with features and benefits of each subscription level and determine which services you want to spin up in your tenant. Once you've done this, you can move on to identifying your use cases to get a better understanding of how many of each license type you need and how you're gonna configure your tenant to support your users.