Magento uses a common relational database anti-pattern called EAV(entity-attribute-value) which has inherit scalability and performance flaws. So in that regards the answer to your question would be Magento will will require increasingly more hard-ware behind it as the database/number of products increases. Though that is something that you are likely going to find with any generic ecommerce platform. Most of them use EAV because of the power it offers in flexibility without modifying the database structure. Most of them can get around EAV flaws for small to medium sized sites using various levels of caching. However, it is no where near ideal and is the price paid for using generic ecommerce software. If you already have a site running on Magento the simplest option is to invest in more hardware resources. Hardware resources *can offset the load/performance of excessive caching and database queries inherit to all the major generic ecommerce packages. far from ideal but probably costing significantly less than building a proper custom solution with a properly normalized database avoiding eav and excessive database queries.