1 Jn 2:18 "Little children, it is the last time: and as ye have heard that antichrist shall come, even now are there many antichrists; whereby we know that it is the last time."
--John does not say there is just one individual that is AN antichrist (one individual) but says there are MANY antichrists> plural.
--John says there are many antichrists "now" contemporaneous with John in the first century.
--when John uses the singular 'antichrist' he is referring to a general disposition of an unbelief in Christ and Christianity and says there are many with that disposition "now".
1 Jn 2:22 "Who is a liar but he that denieth that Jesus is the Christ? He is antichrist, that denieth the Father and the Son."
Anyone (not just one particular individual) that denies Christ is an antichrist (singular). Again, "antichrist" (singular) is used as a general term for the disposition anyone has against Christ.
1 Jn 4:3 "And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world."
Anyone, everyone (not just one individual) that confesseth not Christ has the disposition of antichrist (singular). That disposition was already in the world at John's time.
2 Jn 1:7 ASV "For many deceivers (plural) are gone forth into the world, even they (plural) that confess not that Jesus Christ cometh in the flesh. This is the deceiver (singular) and the antichrist (singular)."
The plural "deceivers" and plural "they" refer to the singular "deceiver" and "antichrist". John refers to these plural deceivers as "the" antichrist and not "an" antichrist of there just being one individual deceiver.