It depends on your portfolio. For one is agency A a low earner for the other is agency A a top tier. Everybody should make one experiences and should then decide, which way to go. I've tried a lot of the whole range (top- and middle tier and also the low earners). Now i'm uploading more and more to macrostock-agencies.

At one stage I was with 6 or 7 agencies.
Now it's down to just 3 and I feel more comfortable with that.
Also, less work involved.

Currently on 6. Also considering stopping BigStock. I'm on a little of all tiers, still experimenting. 

Two = SS and IS that I continue to send in new work.

I left my pictures on BigStock and StockXpert/ThinkStock, which are owned by the same companies, so maybe the answer is four? FT and DT I had personal issues with their changes in policy, and dropped them. Some folks do very well and are happy with those two. But I think once you read the messages, you'll find that most agree, the top four are where you'll make the best return for your time.

SS
IS
FT
DT

That's about it for micro! (opinion) 

ps if someone uses my SS link below and actually uploads and gets a photo accepted, you'll be the first! I find that interesting after three years that, one time, one person signed up for 123RF and did become active. All the rest have been interested and never uploaded one photo. Not begging for a referral, just being amused at the number who come and ask and visit the site promoting Microstock that I created. (spam alert) And never one has gotten past the looking stages into an active uploader. Must be harder than I thought? 

I'm selling my works on 6 agencies.
5 microstock (Shutterstock, Dreamstime, Fotolia and iStock) and 1 macrostock (Alamy)

My best selling agency is Shutterstock.
I might consider joining other mid-earners only if they will pay to have my portfolio as Veer did. Otherwise it is not worth it.

I use over 20.  Started off with the top 4 sites and added more after I started getting regular payments from them.  It has worked out well for me, the smaller sites don't make as much but the money adds up over the years and they usually pay a better commission.  I would of lost a lot of money just sticking to the big sites.

How much is a lot over years how much money sites years for how much work. in round numbers. How many pictures do you have to make it work?

I like the fact that most of the smaller sites pay higher commissions and they might provide some competition to the bigger sites, if they can get enough images.  Sticking to the top 4 sites gives them too much control and we can already see where that is going, lower and lower commissions.

+1

And for some of these sites (like Yaymicro) the effort really is close to zero, just upload via FTP and you are done.

I like the fact that most of the smaller sites pay higher commissions and they might provide some competition to the bigger sites, if they can get enough images.  Sticking to the top 4 sites gives them too much control and we can already see where that is going, lower and lower commissions.

Excellent point! Selling on the top few sites is of course in one's best self interest...today... or maybe even for the next few years. However we've seen what happens when one company seems to think it can control the marketplace. IMHO it's far better to also work on strategic diversification in order not to become a total pawn in any one company's game.

Currently I use 40, but I think it�s enought to use only the TOP10 of the microstock agencies.

I upload to 13. TOP3 earns ~80%.

Another Old Thread Alert, from 2010, previously revived in 2014.

At least its still relevant (well kind of) for what its worth I think the market is more polarised and personally I've stopped uploading to a few sites...Panther Media, Cutcaster spring to mind.