Hi @Artem_Arustamyan and welcome to the community!
No matter the plan type, you can send any number of daily emails to collaborators that have verified emails for their Airtable accounts. Currently, bases on the Free plan are limited to only emailing collaborators with verified emails. For non-collaborators, bases within workspaces on the Pro plan can send emails to 100 unique recipients per day. You can view your current base’s limits at the top of the “Send email” action.
Concerning the number of Slack messages; it doesn’t say in the support page. So maybe no limit or someone here will know or you could ask via firstname.lastname@example.org.