Can I have multiple accounts?

Yes. You can. According to article https://support.chess.com/article/596-can-i-have-multiple-accounts, multiple accounts are supported. But adding some constraints would be good as per my view. second account can play only -200 < x < +200 rating until they play 'n' number of games. Also it would be good to publish authorized second accounts at some place. Everyday I see some 1500 guys playing like a pro and it would be really hard to gain rating with them. It's really annoying when second accounts come to play and steal a lot of ratings. 

Some things to touch base on:

1. Chess.com may or may not be too lenient with this second account thing, especially for variants. For lower rated players, they should not be allowed a 2nd account to fool around on. For higher rated players, their accounts always start at 1500, like with everyone else, and that’s the problem. 
2. Often I see “restart” accounts, where someone closes account and then makes a new one, which according to a Support article (I will find it if anyone asks), is not abuse. While not against the rules, I personally find it a bit immature when you can request a formal speedrun account…

YES Suneth, it's unfair, and I've been victim of that a couple of times as well. Whereas the issue is not very difficult to address. I've already proposed to LIMIT the max. change in rating after a single game.

https://www.chess.com/clubs/forum/view/some-new-ideas-about-rating-calc-ffa

I've just lost more than 50 pts of rating in just TWO GAMES because of that (a 2700 guy who is "1800" lol). It is written "allowed alt". I would definitely request explanations how come that could be allowed, it's totally unfair as players lose MORE POINTS than they should've.

*a 2100-2200 guy is who is "1900"

(It would be much different if a player like me was smurfing at 1900, yes)

If you don't wish to play with them, play semi-anon or non-anon and block that player.

I don't think semi-anon / non-anon is also working fine. I am getting blocked players in the game even in non-anon games. May be it is a bug.

lol this is all against me sadge

I just know it is totally unfair.

Our ratings should reflect our real force. And what we earn or lose should be function of the real level of the players involved, not of their FAKE ratings. 

It's absolutely unfair.

Over the last 7 days I've seen THREE (!) accounts with 0-20 games, playing like 2500-2800. Two of them confessed it were ALT or new accounts. 

Another one, with just 3 games, has just got +95 pts (!!) in a blitz game.

Why is it allowed? It is absolutely insane. It's unfair for people who lose their rating much faster because of that: as 3rd, I've had -17 pts for instance, whereas if the rating of the winner reflected his real force, it would've been -5 or -6 maximum.

(It's also unfair when you win against: I've just got +6 pts, whereas it should've been +10/15 as it's a much much stronger player than his 1500 show).

I agree with what Indipendenza says - second accounts are damaging and untruthful.  They lower ratings and are unfair to anyone trying to honestly improve their game and rating.   I fully support changing the rules to disallow second accounts.

DM me the usernames. They may have been banned previously for alt accounting, and it's not a 2nd account.

Done.

One of them is shown as "closed for abuse". Well, that's good. But no one will reimburse me the points he made me lose (quite a lot).