what is the server is more than that that, and adding more sites soon, I need to increase the 1024m.
No you don't, why would you?

Your 64 GB Ram is devided or better said, it's used by multiple things, so also apache and mysql/mariadb etc. make use of that RAM.
It's not like "the more sites I have, the more of 1024 RAM I need".
There is a way to make it able by customisation that a customer can choose more than 1024 M, but I would advise against that.
For example, when you look at a heavy used forum like this one, often 256M is already enough or maybe 512M.
I've got a server with a lot of Wordpress sites and some forums and the server is running fine and the Wordpress sites just use 128M and some 256.
Suppose you have 10 sites and you all give them 1024M and they then it's already 10 GB. Like explained it can eat up your resources.
Very very little sites need 1024 M RAM.
That I am not able to get it clearly why the global is limited 1024 and how can I increase it.
It's like
@jeroenlaylo and
@Ohm J already explained how it works and also that it can be dangerous for your server. You can increase it globally but there is no reason to.
We have our servers not really very full with users. So for example one server has 88 users which have 229 domains. Some are testing or empty so lets say of those 229 around 180 have a website and we also have a global of 128 and only some users have more.
Suppose 88 users and let's say 80 have a website and everybody would be used at the same time. It's 80x1024M is 81,9 GB and your server is stuck.
So with 128M it's 80x128M=10 GB in use and your server still keeps running.
So suppose you give some people who really need it some more and the rest you keep at 128.
It's RAM which is used by php-fpm, it's not like with a VPS where every users needs to get a certain amount of vCPU's and RAM.