Honestly, the number of users won't really affect much. We should, like Spaz says, increase the number of trainers. This way we can still have high quality players even though our headcount is still low.
Unfortunately it does
The number of active users impacts on draft. As many there are, as better chances you have to pick some good talents. With new incoming draft system, where even bots can have a good pick, I am VERY worried about my 12 managers... there are 4 talents I will loose!!! Note that in my last draft I had ONE 200cm+ potential star 18 years (not to mention his skills...). So, community is critical at this specific stage.
When training, you don't need a large community but a solid training and scouting system. You have to be very careful, patient, persuasive, helpful to get the results you want. Somebody simply has never used monorole training. Others don't know which prioirities can balance a great player with a sustainable salary and team. Sometimes, you have to convince managers to sell their players, although sad to both (you never know who will buy him, my SA Center 21 yrs allstar was sold to a guards trainer...).
In many cases, a good scout sustains you in taking a right decision, or giving you bits of experience. When I picked my Scalambrieri, I started to train him on the wrong way: without my great scout, Scalambrieri would have been a lost talent and not one of top 5 PG's in Italy (despite his potential!) (BTW, don't look at his salary, there is a trick!).
Last but not least, if your player gets a pop every week with a level 7 trainer, but plays 120 mins... well, you don't need him! And it happens too.
With South African guys, I spent half of last season to have my top 10 well trained (a couple were greatly trained, one was proceeding on a mythological path ... ), and since then we are still working to have ALL interesting players well trained. It is a patience game, long, boring, and full of results. I decided to arrange a staff as large as whole SA BB population!!! It is not to have a big team, is to be laser focused on each single player. You can dramatically increase your team evaluations, but to be competitive in a large region takes much time.
I know that HK has plenty of users. Good to them, but community - at least in BB forums - is not so active, so I don't know how many will train correctly their players. 100 users is anyway a good basis to start - of course, as larger as better!
As I am doing in SA with new appointed NT coach, we are working hard to increase the community. So, the effort you are spending on that is a must. Not easy to take results out of it (if the community has a size, there is some reason behind...).