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It has been already said that the eldritch blast is an attack with separate spell attack rolls on each beam
The PHB states:
You can direct the beams at the same target or at different ones. Make a separate attack roll for each beam.
But what happens when you ready an action with an eldritch blast? You can't do multiple attacks on a readied action, I can't find any rules that contradicts that the eldritch blast can be used at its full potential on a readied action.
The Ready action includes no restrictions on the number of attacks you can make. You might be thinking of the restriction in the Extra Attack ability:
Beginning at 5th level, you can attack twice, instead of once, whenever you take the Attack action on your turn.
So a character with Extra Attack can only make a single attack when they Ready the Attack action, but there is no general restriction on multiple attacks with the Ready Action.
For the Action on your turn in the round, you take the Ready Action (PHB 193), establishing a trigger for which you will Cast A Spell, Eldritch Blast. When the triggering situation you specified occurs, you use your reaction to cast Eldritch Blast, which counts as a single action. Eldritch Blast, or its friend Scorching Ray, have you make an attack roll for each sub element of the spell. You are still only making one action, it is just that each component of that action requires its own roll to determine if it hits.
Caution: per the RAW, Readying a spell requires Concentration (PHB 203), an expensive resource for a Warlock. You can only Concentrate on one thing at a time and you risk losing the spell if you take damage (CON save with a difficulty check of 10 or half the damage, which ever is greater). Weigh readying Eldritch Blast against the many Warlock spells requiring Concentration.
