how many EAs can be run at the same time ?

basically yes but on a different chart (can't run 2 EA's on the same chart) depends in the code

it seems there will be error if more than 3 EAs running at the same time.

https://www.mql5.com/en/articles/1412

"Better to say, only one expert (script) can trade at a time. All other experts that try to start trading will be stopped by Error 146. This article will find solutions for this problem."

no error will be if u know how to code as i sad above "depends in the code"

(i have the same EA on different charts working just fine)

Sergery there may be practical limits on how many EA's you can have owing to considerations of stacks and ram and so on but I know of no hard-limit within the MT4 platform itself.

I trade 22 currency pairs with 22 EA's (each assigned to their own chart) within the same MT4 instance. As qjol points out there are things you have to have your EA manage, trade_context contention is one of them, but these are straightforward and well discussed in the forums going back for years.

thanks to qjol and 1005philip !

I need to study further !