Do all users need to use the same API Key?
Using 1 API key is totally fine. It's really just an option, if you so choose, to enable API access for multiple users. Linked to an individual user, you can just see which user is making what calls. While organizationally, you wouldn't have the same level of granularity. In most instances, this is probably not necessary. Its most common use case might be something like Zapier.
Example for users using their own unique API Keys:
Bob wants to connect to Salesforce
Dan wants to connect to Hubspot
Both users could use their own unique API keys to set up what they want.
Learn how to find & manage your API Key below!