There are few lot size limit restrictions to improve our simulated market conditions and mitigate potential insufficient market depth for processing orders.

Maximum Ticket Size is 50 lots, for XAUUSD, it is 20.

You still can open more than 50, just in multiple orders.

There is also a limit of 100 orders you can enter at a time, the server request limitation is 2000 (modifications of TP/SL, orders modifications) and the maximum position limit per day is 2000 as well.