Post study work visa

Have a few questions regarding the post study work visa as my study contract ends in August.

1. While applying for the visa is it necessary to be showing funds for the entirety of the 18 months? Or can I show funds enough for 10-12 months?
2. if i show it only for 10 months and am given a visa only for that duration is it possible to get a further extension until the 18th month? Or is it not possible?

Any and all guidance is welcome. Thanks in advance :)