In 1 Corinthians 10:8, the apostle Paul writes that God killed 23,000 Israelites in one day for committing sexual immorality.

1Co 10:8  Nor let us commit sexual immorality, as some of them did, and in one day twenty-three thousand fell;

The account that Paul is apparently referring to is found in Numbers 25:1-9, where it states that God killed 24,000 Israelites.

Num 25:8  … So the plague was stopped among the children of Israel.
Num 25:9  And those who died in the plague were twenty-four thousand.

How can the two figures of 23,000 and 24,000 be reconciled?  The simplest answer is that 23,000 died in one day, and that an additional 1,000 died on other days. 

Some might ask, how did Paul know that 23,000 died in one day?  Paul received the gospel through “the revelation of Jesus Christ”.

Gal 1:11  But I make known to you, brethren, that the gospel which was preached by me is not according to man.
Gal 1:12  For I neither received it from man, nor was I taught it, but it came through the revelation of Jesus Christ.

So it is quite possible that Jesus Christ revealed this to Paul during this revelation.

So, Did God kill 23,000 or 24,000 for Baal worship?  The answer is that God killed 23,000 in one day, but killed 24,000 in total.