When you've got a lot of people, you've got a lot of changes - that's the long and the short of it. This is one thing the check in/check out model (as exemplified by Perforce, among others) is really good for managing. When you go to check out a file, you find out straight away if someone else has it checked out.

If you're just going to make a quick tweak, you'll probably risk it. Either they check it in first, and you've got a very minor merge, or you do it first, and they've got a similar minor merge. Not a big deal, in either case. (And when your gamble doesn't pan out, tough luck. No sympathy from anybody. You knew the risks.)

But, if you're going to make a very large, sweeping change, you'll probably be a bit more cautious. And that's fine: you can go over and talk to them, or message them, or email them, or whatever, to find out what they're doing, and coordinate your modifications appropriately.

I've literally never once found this less than somewhat useful. It's, like, the source control analogue of static typing: a huge pain in the arse if you're not used to it, but, if you've seen it play out, it's a mystery how anybody gets any work done in its absence.

(Of course, if you use git, maybe you can just email/Slack/etc. everybody on the team before you go to edit a file, just in case, and then wait for anybody to mail you back before proceeding... well, I don't deny that would work, assuming everybody checks their mails/Slack/etc. regularly enough. After all, I hear people get work done in dynamically typed languages too! But just think how much better things could be, if the version control system could look after this for you!)