I have 21 tests, which range between about 4,000 and 35,000 matches. There are a number of factors, the main one being where your ancestry is from - most tests have been taken by people in the Anglosphre.
It depends what you want to do. You can use DNA to prove your lines back, break down brick walls, discover offshoots born out of wedlock and more. As an example, my best friend's father took a DNA test and I quickly saw in his results lots of familiar matches. There was clearly matches that showed a descent from two of my ancestors. After triangulating with other tests I was able to determine that one of my ancestors born in 1803 had two children born out of wedlock to a married woman (by infidelity). Those birth ended in 1841 when he moved about 20 miles away. So, a little extra to put on his bio / rap sheet.
If you want to utilise matches to do the above, the rough guide is:
1) go though all your matches and add them into groups based on your sixteen gg-grandparents - you will find some listed as having a common ancestor (if you have a linked tree on Ancestry) - use those as a starting point to add people to groups - but then later used shared matches (under that tab) to group people who match with each other into common groups
2) also add people into groups based on shared matches when you can't see the link; currently Ancestry only gives you 24 groups, which is annoying as you need 48+ groups for all the matches - they will be adding 64 soon - at current I add a note to others like - #OVERFLOW - PAT - WELSH65#
Here is a brief video on the topic -
https://invidious.privacydev.net/watch?v=81OsvG81SyUOne issue with Ancestry is that is only shows shared and triangulated matches >= 20cM. So if you have 20,000 matches, you'd need to go through all of them to find all the deeper matches. So I have developed a tool that just identifies the ones with shared matches, which are the most useful, as they show a flow of genes. It's important to only consider matches when you can see a common flow of genes and ancestry together. You will probably get some matches where Ancestry is flagging a common ancestor, but the slow of genes from shared matches does not overlap. You can also get some matches with no shared matches that have a common ancestor - in which case you can't be sure if the shared DNA correlates with the Ancestry.
I also have another tool that downloads all the trees of my matches and looks for common surnames, places and individuals. That is key, as you can have huge clusters. I have some with about 700 matches and it can be difficult to find a common thread in them.
This is useful for groups of matches for whom you don't know how they relate to you. You have created a group based on shared DNA. Now you need to pick out matches who share the same ancestors. Once you have that - that can give you a coordinate of people who are your ancestors or close relatives.
If you already have a good tree, these unknown groups could be a part of unknown ancestry, ancestry by a non-paternal event (e.g. infidelity), or descendants of one of your ancestors born via a non-paternal event.
Going down this route is very heavy work, but IMO the most rewarding work in genealogy. I still have multiple missing lines and lines I haven't confirmed with DNA. I am fairly sure I have three ancestors who were born via infidelity, which is around the average (according to studies). Of your roughly 128 ancestors for whom these DNA tests are useful, you'd expect about 2-3 to b born of infidelity. But generally you need quite a lot of tests to solve these mysteries.