Hello,
If you use platform manually there is no limits.
There are limits for strategies. Strategy can't place more than one order at the time(in one second), if it's instrument and trade side are the same.
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is it true that only one trade is allowed for all currency pairs at the same time??
No
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Be I'm wrong or is that fact useful for automated trading??
Yes, this allows to avoid spamming. For example, there are cases, when some strategies places pending order and then immediately cancels it and do it in a loop.
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Could it be or is it a fault at your side??
No