There are no known limits instead it is based on the resources and specs per server not based on lpanel itself. Lpanel can support as many servers and clients as you can put in it. Unlimited is not a number but the idea is that it can support as many as you want to put into each license.
dual xeon server 2.8+ Ghz 400GB
can support up to 1000 clients Lpanel will not lagg or slow down even if you packed in 4000 users as it is not Lpanel that will craw instead it will be the server.
You can have 20,000 servers each with 1000 clients and run all of this via one License of Lpanel. I have not seen a client base this large however this is based on larger confirmed client bases being managed and run from a single license...