Google API key is now required for new websites even for a single map embed.
How are you going to handle this?
I guess the correct way should be to ask a client to create his google API key and send it to you.
The first problem is that not all my client has a google account, some doesn't have one and some have a personal account but not a BRAND account.
Even if they have an account the often get stack with really simple tasks.
The other option is ask for their login and password and create a key for them but thats would give me also access at gmail account, if two step verification is enabled it would be more complicated to do it.
The other way could be to create a single account and a key to share between sever website, not sure if this is OK with google maps policy and if the account reach the max daily view all websites maps will get obfuscated.
As far as i understood multiple keys on a single account will sum page view and it wouldn't make any difference using a single key across multiple websites.