You'll eventually be locked into a permanent Affinity by the end of Episode 8. If you make 5 or more Major Decisions of each type you'll be stuck with permanent Neutral Affinity. So on a technical level, you can make up to 4 DIK Major Decisions 'safely.'
That said, there's a little more to it. First, when you make the decisions can matter; if you make them all at once, you could easily swing over to (temporary) DIK Affinity, which might screw you over in the short term (e.g. Jill's path has frequent checks to weed out DIKs).
Second, the Major Decisions themselves can potentially have their own consequences. Fighting the jocks in Episode 2, for example, is necessary to fight Caleb in Episode 5 regardless of your Affinity. So you should probably put some thought into how those DIK decisions might come back to haunt you even if there are only a few of them.
Ultimately, it's going to come down to what sort of playthrough you're looking for. So my advice is to not to think of it as simply a 'CHICK' playthrough, but to pick some overall goals and 'style' for the MC, then keep them in mind when facing tough major decisions. Or just have more than one playthrough and cover all the bases.
