Everything changed on Jan 1 2025. There is no longer a discounted carnet of 10 tickets. Those t+ tickets are no longer available although apparently there is the occasional machine that still has them.
A single RER/metro ticket loaded on a Navigo Easy card is 2.50; a bus ticket loaded on the Easy is 2 Euro -- theyi are no longer interchangeable. You can load as many as you think you need and load more if you run out. Or you can load a day pass for 12 Euro if you will use 6 or 7 rides in a day which would be unusual.
The Navigo Decouverte is a different card which takes passes (but not tickets) and you can load a 31 Euro weekly pass on it which is one of the great travel bargains in the world if it fits your dates.
Anything in guidebooks etc before this January is now obsolete.
2.50 tickets
Note that these tickets for metro/RER will take you anywhere in the Ile de France, there are no more differently priced REr tickets depending on destination.