Each FTMO Futures account has a maximum number of contracts (the Max Contracts limit) you can hold at one time. On the Evaluation, the limit is 5 contracts on a 50K account, 10 on a 100K account, and 15 on a 150K account (the same for Growth and Pro).
On a Sim-Funded Account, you start with a lower base limit that can increase up to the same ceiling as you generate profit. More information about the Max Contracts Scaling can be found on the Trading Objectives and Rules page, in the Maximum Contracts section. Micro contracts count towards the limit at a reduced rate (1:10) compared with mini contracts.
If you try to exceed this limit, the platform will not allow you to open a position.