Hello @Massi,
Welcome to the Klaviyo Community!
I think taking a look at our Understand multi-account user privileges Help Center article could be helpful in answering most of your questions.
In essence, your login Klaviyo credentials will support multi-account access. This means that you can access multiple Klaviyo accounts using a single set of credentials that you’ve created for Klaviyo. In order to access these other accounts, the Klaviyo account owners (or admins) will need to grant you access and invite you. So to answer your question, you’ll only need one set of Klaviyo credentials. Even if you were working with a large number of clients.
As for your second question, it typically would not be your email that appears when sending actual campaign or flow emails out to recipients. In fact, we recommend using a sending email address that includes the business’s domain name. This indicates to inbox providers that you are a legitimate sender with a legitimate business. Oftentimes, the sending address used will be one set through the account as mentioned in the How to change your sender email address Help Center article.
If you’re just getting started with Klaviyo, you could also find some really helpful guides and tutorial through our Getting Started with Klaviyo Academy course too!
I hope this helps!
David