I'm not really sure if its possible to answer this question, since no specific information regarding the situation you intend to deploy is given. But I'll try regardless. Hope its helpful.
MQTT is a protocol not a solution, the size of the deployment entirely depends on the following:
- number of clients
- size of the messages being sent
- rate of message being sent
- type of broker (single instance/distributed cluster)
- type of network infrastructure involved
1000's of clients is nothing compared to some of the deployments from tech giants like Microsoft and IBM.
Your best bet is to build a test rig and test it to see how much resources your specific scenario can exploit.