Hi Faisal,
For failover to work, both devices must be the same model (ie: ASA-5510), and for the 5510 specifically, both units must also match on having (or not) the Security Plus license.
Therefore, if you are purchasing the ASA-5510-SEC-BUN-K9, (which is the 5510 with the Security Plus license), then you need two of those.
I hope this answers your question.
Sincerely,
David.