Note: there is a page about Slack for ASF project people on infra.a.o.
Slack has three types of accounts, Member, Multi-channel guest, and Single-Channel Guest. Member and Multi-channel are pretty much the same thing, at least in cost to the ASF. Those accounts count towards our monthly usage if there are active accounts. Single-Channel guest accounts do not cost the ASF under any circumstances, however there is a limit of 5 guest accounts to 1 paid account. At the current rate, we'll never hit that limit of accounts.