The maximum number of atoms a molecule can have is theoretically infinite. However, in practice, the size of molecules encountered in nature and synthesized in laboratories is typically much smaller. Large molecules with thousands or even millions of atoms do exist, such as polymers and certain proteins, but they are relatively rare.
The largest known molecule by mass is a protein called titin, which is found in muscle cells. Titin has a molecular weight of about 3.2 million atomic mass units (amu) and consists of around 244 individually folded protein domains. It is worth noting that the size and complexity of molecules can vary greatly depending on the context and the specific chemical compounds being considered.