This is a great question! Unfortunately this is not something that can be quantified. Before I answer your question, I need to make you aware of a few points. MatrikonOPC Explorer is a free test tool (OPC Client) designed for diagnostic purposes. It is not designed nor qualified for long term production use.
That being said, the restriction you are seeing is unlikely to be in the OPC Client. This behaviour normally highlights the maximum performance of the OPC Server that the OPC Client is connecting to. To increase this performance, you can look at adjusting the update rate that was set in the OPC Group within OPC Explorer. The default is 1000mS and 500 tags at 1000mS may be too much for your OPC Server to handle. You may like to increase this update rate to 5000mS and see how many tags can be obtained before you see the same behaviour. The performance and resources of the PC where the OPC Server is located should also be considered as well as network resources if the OPC Client is remote to the OPC Server. If the CPU, RAM or network traffic is heavily loaded, this will impact the performance of the OPC Server. You also need to consider the communication between the OPC Server and device(s). If you are using a Modbus OPC Server and the communication to the PLC is a Modbus serial link, the maximum reliable tag count you can expect is about 300 tags per second. This is due to the restriction caused by the serial port.
I would also suggest working with the vendor of the OPC Server to determine the maximum expected performance from the OPC Server so you know what is possible from the product.