There is no hard and fixed rule on the number of ETP's you would present to any given route. The number of ETP's is directly related to the number of ETOP's alternates required for the sector in question and the number of ETOP's alternates directly relates to your ETOP's certification.
For example. (Great Circle routes used)
London to New York (on a 120 minute certification) would require 3 ETOP's alternates, EINN, BIKF & CYQX. If I was dispatching it, which is unlikely at the moment, I would be providing ETP's for at least EINN/BIKF, BIKF/CYQX and EINN/CYQX.
For a more southerly routing, for example London to Montego Bay, you would need 4 alternates EINN, LPLA, CYQX and TXKF. ETP's would be provided for EINN/LPLA, LPLA/CYQX, CYQX/TXKF, TXKF/MKJS.
Moving even further to the south, London to Barbados gets a bit more tricky and would probably need 180 minutes certification or (and preferably) at least another engine!!
Common sense is the key to good dispatch practice.
Whooaahh!!!