Can I refuse extra business trips? My contract allows only two per month.

Short Answer: Yes, you can refuse if your contract limits business trips to two per month.

Full Answer: If your employment contract specifically states a limit of two business trips per month, you have the right to refuse additional travel without breach of contract. When discussing this with your employer, you should refer to the relevant clause in the contract.
However, if the contract is variable or includes a flexibility clause, the employer may require more travel. In such cases, you may need to negotiate or seek legal advice. Make sure all communications on this issue are documented.
Regardless of whether your employment contract is written, implied or oral, you have the right to take legal action for breach of contract. This applies if your employer has wrongfully dismissed you or refused promised or implied benefits. You will need a lawyer who is an expert in employment law to represent you in your case.

General Federal Standard: Employment contracts are binding if they do not violate labor laws. Employers must adhere to the agreed-upon terms unless both parties consent to modifications.