By design, almost every fighting game combo worth doing involves attacks which are unsafe if blocked. Otherwise everyone would repeat the same motions mindlessly no matter what.
Because fighting games are built to be highly interactive, being able to confirm whether your attacks are connecting is a critical skill. When you’re trying to combo into super moves, proper hit-confirming enables you to revert to a safe block string and avoid disaster.
What’s the best way to hit-confirm? Well, there are several equally good ones, so it boils down to a matter of personal preference. The important thing is to be realistic about your limitations and construct your gameplan to accomodate them.
In general terms, everyone should be able to react to three light attacks connecting – in time to cancel the third one into a special or super move. If you bring that number down to two light attacks, i believe that almost everyone should be able to react to that as well. It’s not exactly easy, but it is a very realistic goal if you practice with dedication and play with focus.
In special cases, it’s also possible to react to a single attack connecting in time to cancel it. However, this requires either someone with truly exceptional reaction time, or an especially lenient combo window such as the extended cancelability period of 3S Chun Li’s c.MK poke. At any rate, it’s generally not practical to worry about hit-confirming single attacks.