If you're posting a thread on here, numbering its posts can be a good way of indicating they are part of a thread. People may see individual posts shared out of context and not realise they are part of a thread, which is why numbering can be useful.
You can number simply by just putting 1,2,3 etc at the end, or 1/3, 2/3 etc if you know how many posts there are. Some people put 🧵 next to numbers to indicate a thread.
More info about creating threads at:
➡️ fedi.tips/how-do-i-post-a-thre…
How do I post a thread on Mastodon? | Fedi.Tips – An Unofficial Guide to Mastodon and the Fediverse
An unofficial guide to using Mastodon and the Fediversefedi.tips