Does the following sentence make sence to you? Is it natural for native speakers? If not, please help check it. Thank you.
There is evidence to suggest that a great number of people retired spend more and more time on voluntary charity work, which they are very happy to take part in.
Just one correction. It should be "... a great number of retired people". There are no other grammatical errors although it's a bit wordy and I think it could do with being tidied up a bit. You might also want to say "a lot of time" rather than "more and more", since you don't say "more" than what. More than they used to? More than non-retired people. I think it goes without saying that people are generally happy to take part in voluntary work or they wouldn't do it.
Here is a condensed, neater version.
There is evidence to suggest that many retired people are happily/willingly volunteering a lot of their time for charity.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.