If you have many donations, support can enable automatic thanks. One is for the creation of thanks emails. The other is for the automatic sending of thank you emails. Use the donations monthly report for this. Two buttons to do this will be available for you. The first, create missing thanks, will generate all missing thanks for the month.
You could send them then, by clicking the send unsent thanks email. As mentioned, you can personalize any of these thanks first. It would go a long way to showing your donors you appreciate their giving.
You can also send donation thanks individually.