Someone in the irc channel #windows told me how to achieve this.
For anyone else wondering: Just increase the Windows desktop heap.
Same question.
I can start 32 MT4s.
8 years later I can reply to Xennon based on my case
In my case I need to launch many MT4 instances, at least at my max CPU threads, to run thousands and thousands of backtests with Tick Data Suite to check my EA's performance (money, not speed).
I'm developing a Symfony project (a command line component) to launch a lot of backtesting to test your EAs, you launch a single command like this "bin/console metatrader:backtest:generate Prudencio .SPAINCash H4 5000 2013-01-01 2022-01-01" and the system prepare a tester.ini with a Prudencio.ini file (Prudencio is the name of one of my EAs) to automatically start metatrader, with Tick Data Suite enable, run the backtest, store html report, load and parse the html report and import to a database for analysis.
Because MT4 is a single thread the backtests take a loooooong time to run in a 64 CPUs machine, that is what I need to launch as many MT4 instances as many threads my CPU have, for optimizing the time and do more backtests in less time running in parallel.
This is probably the most use cases to launch many MT4 instances with a big CPU core machine, to use the max power of your CPU with many EAs or indicators, I guess.

- Free trading apps
- Over 8,000 signals for copying
- Economic news for exploring financial markets
You agree to website policy and terms of use
Dear traders and MT4 support,
currently, I am running many MT4 instances on one pc.
However, there seems to be a limit of the instance count.: For me, I can only start 18 MT4s.
What's the reason for this and can it me changed?
Neither my RAM nor my CPU is anywhere near its maximum.
best regards,
Daniel.