Simultaneous calling of url

Hello
I want to call multiple sites simultaneously by xargs or parallel
for example
a.com/srv/1
a.com/srv/3
a.com/srv/2
a.com/srv/n

That means 500 different sites will be called in a moment
Then I use command I call a url at the same time. I need to call each url once, but every 500 sites simultaneously.
Please give an example

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