This is actually simpler than it appears.
In the book of Hebrews, if we ignore the instances where "holy" is used of singular items such as the Holy Spirit, and the "saints", etc, we are left with the plural instances which are:
- Heb 9:8 - By this the Holy Spirit was signifying that the way into the holies has not yet been made manifest, the first tabernacle still having a standing,
- Heb 9:12 - nor by blood of goats and calves, but through the own blood, He entered once for all into the holies, having obtained eternal redemption.
- Heb 9:24 - For Christ has entered not into holies made by hands, copies of the true ones, but into heaven itself, now to appear for us in the presence of God,
- Heb 9:25 - nor that He should offer Himself repeatedly, just as the high priest enters into the holies every year with the blood of another.
- Heb 10:19 - Therefore, brothers, having confidence for entering the holies by the blood of Jesus,
- Heb 13:11 - For the bodies of those animals whose blood is brought for sin into the holies by the high priest are burned outside the camp.
The author of Hebrews is using Hebrew idiom when he refers to "holies" - ie, all the holy places that compose the sanctuary. That is why, some translations translate this "holies" as "sanctuary" which is the sum of its parts. [Note that "places" is not in the Greek text of any of these verses by supplied by the translators.]
That is, "holies" refers to the entire sanctuary.
To my knowledge, the only version that handles this consistently and correctly is the New English Bible.