Yes, you can open a maximum of 50 lots per trade. If you wish to open more than 50 lots you will need to add additional trades.
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The reason for the lot size limit per trade is to prevent traders from experience large volumes of condition decay on their orders. It is far easier to get better fills on 50 lots than it is to get fills on 200 lots.
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It is important that despite the lot size limit being high at 50 lots, each trader maintains responsibility risk management when trading.