The c in -cs can be pronounced or not. There is no real rule I’m aware of but there are two or three times more words where the c is pronounced.

Here are some words with the c in -cs pronounced:

Alambics, allocs, aqueducs, arcs, bacs, becs, bivouacs, blocs, boucs, caducs, chics, chocs, clics, couacs, ducs, déclics, échecs, grecs, hamacs, lacs, laïcs, parcs, publics, rocs, sacs, secs, trocs, …

and here are common words with that ending c mute:

Accrocs, ajoncs, bancs, blancs, brocs, caoutchoucs, clercs, crocs, entrelacs, escrocs, estomacs, flancs, francs, joncs, marcs, porcs, tabacs, troncs.

Both pronunciations are accepted with:

Crics

but nowadays, the c is usually pronounced.

Regional French might pronounce the ending c in porcs and other words like caoutchoucs, clercs, tabacs…

The ending c in zincs is expected to be pronounced g.

Both c and s are pronounced in pacs (from the acronym PACS: pacte civil de solidarité).