I'm new to WAV and the WAV Community.
Hey there @tamla
First off, welcome to Wave and the Wave community
Glad you could join us here. Our existing threads have a ton of good info so at any point you can try searching for keywords in the search bar.
Presently, the amount that you can bill to your customers has no limit. However if you are trying to process credit card payments on your invoices using Wave Payments, our risk department may require some more information from you about processing large payments. These numbers vary depending on your active payments account.
If you're not processing CC payments you can bill for a large sum of money and collect it through a different payment method.
I've seen some businesses with thousands of customers on their account. The more data stored in Wave may cause it to slow down a bit, but this would be unusual behaviour.