What is the amount upper limit on a single PayPal transaction?
As of 2013 - in European Union EUR 8'000, in United States USD 10'000, in other countries in the same range, clarify exact amount with local PayPal.
There may be though other limits on the buyer's side, for instance there is an additional limit for not verified PayPal account in EU, or credit card / bank account limits. Check with local PayPal and/or corresponding financial institution.
Is there a limit on a number of PayPal transactions?
There is no limit.
Is there a limit on the total receipts via PayPal?
After a certain amount of annual receipts via PayPal (EUR 2500 in EU as of 2013), one-time extended account verification may be required by PayPal. Please get in contact with local PayPal for details.