Is there a limit on the number of MNQ or NQ contracts I can buy set by CME or is it determined by the broker, assuming sufficient margin?

The number of MNQ (micro E-mini Nasdaq-100) or NQ (E-mini Nasdaq-100) contracts you can buy is primarily influenced by the margin requirements set by both the CME (Chicago Mercantile Exchange) and your broker. The CME establishes initial and maintenance margin requirements for these contracts, which act as the minimum funds you need to hold to open and maintain a position. As long as you meet these margin requirements, the CME does not explicitly limit the number of contracts you can trade.

However, your broker might have additional restrictions. Brokers can impose their own rules that may be more stringent than those of the CME. They could cap your contract limit based on their risk assessment or your account profile, which includes factors like account size, trading history, and creditworthiness. Thus, while the CME sets the foundational margin requirements, it is often the broker that places specific caps on the number of contracts you can purchase to mitigate risk. Always check with your broker for any particular limits or conditions that may apply to your trades.