Thanks a lot for the replies!
Start with 2 (2 is minimum req. for MESH). If you think you are not getting good coverage. Can always stack new ones.
b/w can still use ur old extender if not wanting to buy another mesh router.
(I'm using mine)
Check if the router they are going to give you is more than 300MBps.
Note, for MESH router it would decrease speed.
Config would be This way
(Base router where your internet cable is to connect-->connect MESH router to it via lan cable(it would configure itself)-->Keep on adding new MESH routers)
Go for a good base router.