We've grown from about 10 to almost 200 in the last 7 years. The first thing that needs to change is that you will need better documentation and more standard processes. Requirements may have to get more formal as well.
You should also consider hiring specialists as you grow. If you have a database backend, you should have a at least one dedicated database specialist. You should probably spend money for a tester.
You will have more projects going on and a bigger need to manage tham, so if you aren't using one now, you need a project management system and a bug tracker. You need to create a deployment porcess and limit production right to only those people who will be doing deployments, no more making changes directly on prod. Your developers will need to be limited to select rights only on prod.
As you have larger teams, you will have more people problems and will be more likely to hire some less skilled people (relatively easy to get three good developers when that's all you have, much harder to hire 30 at one time). Even though you try to get the best people, the more you hire the more likely it is that you will get a dud, so be prepared to let people go as well.
Coordination between people is key. Two teams making mutually exclusive changes to a product is a bad thing.
With only two or three developers you can't afford to have junior people - everyone must be working at the senior level. With a lot of developers, you can't afford not to have junior people. Hire some junior and train them the way you want them trained. It's usually better to work somewhere that has a career path not everone at the same level.
As your team grows many of your current developers will become the new management staff. Some will hate that, make sure those have an opportunityfor a promotion to a senior developer rather than management. Don't lose all your technical expertise to management. Reward those who don't go into management because you need their detailed knowledge of the current system to get the new people up-to-speed.