Purpose – This paper aims to propose the framework of a decision support system (DSS) to select the optimal number of suppliers that are candidate to join a supply chain network.Design/methodology/approach – The DSS bases the decision on the cost evaluation of the transaction among the buyer and the potentially available suppliers by way of a Monte Carlo approach. In particular, the presented DSS includes a statistical module and the DSS core. The former module estimates (in a probabilistic way) the exchange performance indices, i.e. total cost of the transaction, purchasing price and additional costs of purchasing, while the latter module implements the transaction evolution making use of a simulation model. The DSS is tested by way of a case study, namely the supply of a customized product by a general contractor in the construction industry.Findings – The obtained DSS results are validated with the actual data of the purchasing, and confirm the underlying model suitability and the DSS effectiveness for...