It is likely that only one familiar would be able to attack
There are a few things that point to this being the case:
The Pact of the Chain feature uses the singular
The Pact of the Chain feature states:
[...] Additionally, when you take the Attack action, you can forgo one of your own attacks to allow your familiar to use its reaction to make one attack of its own.
Though this feature existed before the flock of familiars spell, the feature didn't receive any errata with the release of Lost Laboratory of Kwalish. Applying the feature as is it consistently uses the singular throughout, which, strictly Rules As Written, means that only one familiar can gain this benefit at a time.
Other parts of the flock of familiars spell only apply to one familiar at a time
The flock of familiars spell states:
[...] Each familiar uses the same rules and options for a familiar conjured by the find familiar spell...
When you cast a spell with a range of touch, one of the familiars conjured by this spell can deliver the spell, as normal...
This is more of a supporting idea; here we see another feature of familiars applying only to one at a time so it would make sense for other features to work similarly.
Note also that flock of familiars never states that the familiars function as some sort of collective unit instead each individual familiar follows the rules of the find familiar spell.
If this were allowed, other familiar capabilities would be able to be done in unison
If all the familiars could attack in unison, then you could also dismiss and recall the familiars all at once; both of these abilities use the singular in their descriptions:
[...] As an action, you can temporarily dismiss your familiar...
As an action while it is temporarily dismissed, you can cause it to reappear in any unoccupied space within 30 feet of you...
So if Pact of the Chain's attack worked with all the familiars at once, so would this ability. Though it's not game-breaking by any means, suddenly summoning between three and ten familiars is certainly a significant buff to your party. This somewhat points towards Pact of the Chain's attack only working with one familiar at a time.
Arguments 2 and 3 are much weaker than argument 1, especially since the flock of familiars spell is somewhat inconsistent: It specifically says touch spells can only be done by one familiar at a time, yet it also says:
Familiars summoned by this spell can telepathically communicate with you and share their visual or auditory senses while they are within 1 mile of you...
And doesn't mention whether this sense-sharing can only apply to one familiar at a time, or even whether it is always on (the version in find familiar requires your action to activate and removes your own senses while sharing). And then flock of familiars doesn't mention the ability to dismiss/recall familiars at all.
The spell isn't consistent in how it addresses the abilities of find familiar and how they change (or don't change) with the flock of familiars spell. As such arguments 2 and 3 are significantly weakened, though argument 1 still stands.
Overall, the use of the singular in the Pact of the Chain feature and the fact that some (though not all) features of the spell are called out as working with only one familiar at a time both support the idea that Pact of the Chain would only work with one familiar at a time.
Note also that Pact of the Chain is not itself part of the find familiar spell, and also Jeremy Crawford, though he was a designer of the book, was not the lead designer like he was for other books for fifth edition. It is quite possible the Pact of the Chain feature was forgotten about when writing the description for the flock of familiars spell.
A small note:
The Pact of the Chain features states:
[...] Additionally, when you take the Attack action, you can forgo one of your own attacks...
You do not forgo your entire Attack action, just one attack that you can make with the Attack action (features such as Extra Attack allow you to make multiple attacks with the Attack action; you could then forgo each attack individually as you wished). For further reading on the distinction between attacks and the Attack action you can read: "What does upper-case-A-Attack action vs. lower-case-a-attack mean?"