As far as I see an extended licence allows the user to use your image on things like T-Shirts,mugs etc.

Supposing they buy an extended licence from Adobe Stock,can my image still be on sale with the other stock sites too or does an extended licence mean that you grant for example Adobe Stock exclusivity?

Extended licences have nothing to do with exclusivity. They just give the customer more right to use in a calendar, or as you said on a T-Shirt and some agencies require extended licences for higher print runs (or least they used to do). So unless the image is exclusive in the first place, for example because you are exclusive at iStock or have exclusive images at Stocksy or some other agency, you can continue to sell the image elsewhere.

Sometimes you here about buyout sales, where you get a lot more money and then have to remove the image from all agencies, but in that case, the agencies usually reach out to the contributor with an offer made by a customer, which the contributor can then accept or decline.

You should read through the license terms for the sites you submit to. It'll help you understand how things work

https://stock.adobe.com/license-terms

https://www.shutterstock.com/license
https://support.shutterstock.com/s/article/Standard-License-vs-Enhanced-License?language=en_US

Adobe stock does a pay per credit thing. You buy credits and spend them on stuff. If you have a subscription level set I think that stuff can just automatically keep buying credits.
Most stock websites have either a pay a subscription fee and download all you want, or a no subscription fee, and pay per use of each item. Both have their uses, I have found that adobe's stock elemetns are very high quality and very well organized, but their browsing experience isn't the bessst.
Generally I look at the monthly subscription fee places first, then if I can't find what I want go to adobe, and if I must lisence the thing for the one time price. If you're making art to sell and generate 100's of thousands of dollars, 10 bucks per image is nothing. If you're making a video for youtube that has no likelyhood of earning any money, 10 bucks per image could be an expense not worth it.