Alice and LHCb are around 1k. ATLAS and CMS are closer to 3k. There are some very cool experiments at CERN that have less than 100 members, which typically take advantage of either existing LHC interaction points[1] or the accelerator chain that feeds the LHC[2].

[1]: https://faser.web.cern.ch/the-collaboration

[2]: https://base.web.cern.ch/content/people