Je voudrais remercier tous ceux qui ont aidé avec l'organisation de l'événement.
How is tous pronounced here? I know that the pronunciation depends on whether tous is a pronoun or an adjective, but I'm not sure what it is here.
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Here, tous is an adjective agreeing with the pronoun ceux so according to the rule you linked to is pronounced /tu/.
Had it been pronounced /tus/, that would have lead to /tus.sø/.
Je veux les miens plus ceux de mon frère (I want mine and those of my brother)
where plus ceux is pronounced /plys.sø/ and:
Je ne vois plus ceux de mon frère (I don't see those of my brother anymore)
where plus ceux is pronounced /ply.sø/.
It's always pronounced /tusø/.
Tous is a determiner here (hence feminine form toutes celles /tut sɛl/), but the /s/ is combined to the one at the beginning of ceux, so it would be pronounced the exact same way whether tous were /tus/ or just /tu/.
The only case I can think of where the pronunciation may be different is in Quebec, for speakers who have only the invariable form /tut/ regardless of what standard grammar would have. In that case, you would have /tut sø/ instead (spelled toutes ceux, but toutes ceusse is more likely when you get to that degree of joual), but this is considered nonstandard even for Quebec French.