We can reasonably affirm that this refers to the Twelve and not exclusively to Peter and John. Here is why:
Textual arguments
1- The immediate context: In Acts 5.12, Luke specifies that many miracles were done “by the hands of the apostles” (in the general plural), and that they were all together in Solomon’s Portico.
2- Verses 5.17-18: The high priest and the “laid hands on the apostles (τοὺς ἀποστόλους) and put them in the public jail.” The definite article “the” and the plural suggest a group, not just two individuals.
3- Verse 5.29: “Peter and the apostles answered…” This wording singles out Peter as spokesman, but confirms the presence of the other apostles.
4- Verse 5.42: After their release, “they did not cease teaching” — again in the collective plural.
Contrast with Acts 4
In Acts 4.1-3, Luke had been explicit: “As they spoke to the people, the priests came upon them… They laid hands on them” — the “them” clearly referring to Peter and John, named in chapter 3.
If Luke had wanted to designate only Peter and John in Acts 5.18, he would probably have used the same precision as in chapter 4, or mentioned their names.
Conclusion
The use of the general term “the apostles” in Acts 5.18, without any restriction by name, probably indicates that all Twelve were arrested this time, marking an escalation in the repression compared with the arrest of Peter and John alone in Acts 4.