Setting Up Partner Access for O365 Clients
You can access all of your tenants from a single login if you have a Microsoft Partner Account. If you don't already have a Partner Account you'll have to set one up then you'll be able to send Delegated Administration requests to your customers. A global admin from each customer must login to accept the delegated administration offer.
Once that is done you'll be able to sign with your partner account > click Partner > click Customers > enter the customer name then select the customer > click Service Management > click Office 365 > now you'll be in the O365 Admin Center for your customer.
If you don't already have the partner account the following article walks through the steps you'll need to create one.
Once you have the partner account the following article walks through offering delegated administration.
Note: delegated administration does have a few limitations. For example, you wont see Multi Factor Authentication settings through delegated administration so if you need to access those settings you have to sign into the tenant with a Global Admin account. It's common that some settings or views may appear differently or be missing when using delegate administration.