Bank statements to show pension for retired dad to visit UK

[Destiny2112]

I am planning on calling my parents or mum over to UK.They have come to the UK before when we had only 1 child.
Me and my husband have 3 children now and living in a 3 bedroom house same as before.

When applying for a visit visa for parents, can they reject because of lack of room in our house now.
We are happy sharing bedroom.

Please advise.
Thank you

[Destiny2112]

How many months of bank statements of my dad do I need to show for application ?
He has pension going in.
Is 3 months enough?
What is the criteria to be successful?
TIA x