Is a 1956 penny with the "one cent" back side rare??

They are not that rare.

Not rare.

Just less common that a 2019 penny

Then it depends on the "mint mark".

1956 (no mint mark) - most common

1956 D - many, but less common

1956 S - the least produced of 1956

Plain (no mint mark) is the Philadelphia mint.

D is the Denver mint.

S is the San Francisco mint.

Do some reading online about coins.

Depends on what you call rare. It's worth more than $0.01. it could be worth up to $10 to the right collector. It would have to be in mint condition and to the right collector. Is it worth more than a penny yes it is. It really depends on the mint marking and who you sell it to

They aren't rare at all.

No it isn't.