Depends on how large the filter is, & how well it is cycled. Normally, maybe 3 or 4 would be full. For a temporary situation, heavily overfiltered, well cycled, with a couple large water changes per week, I'd go with no more than 10.
Only 3 guppys max, its not alot of space and you run a risk of the guppys harrassing each other- either go for 3males or 1male and 2females, the tank will need a filter and heater as well.
Actually, if I had that tank and wanted to put Guppies in, I'd go for 3 females. Then there is no risk of them being harrassed by the male, and they should be able to live peacefully together. With all males, or males and females, there could be some aggression, and in that tank there would not be much room to get away from the aggressor.
I need to rise my GH from 2 to about 10 and pH from 6.5 to >7. We don't have those hardy guppies here. Only very far bred sensitive ones. I have 7 guppies at the moment, 2 males and 5 females.
I have danio chopraes (8+5 fry), panda corys (6), mountain shrimps (2) and amano shrimps (4) in it.
Can you possibly get Feeder Guppies? They are generally a lot hardier, since they haven't been incredibly inbred to be fancy. Also, the p.H you have should be okay for Guppies, even Fancies. The GH should be higher though, as you said.
That stinks...what I usually do is breed Feeders with Fancies so we can have hardier, less Fancy Fancies. They still come out very pretty, just not as ornamental.
Anyway, I think 7 should be okay. They aren't huge, and don't produce a large amount of waste.
OK so I know this is the very wrong answer. But how long are you thinking the shrimp will need? If it is just for a very short time. Yes seven will be cramped, but just keep an eye on the water parameters, and watch to make sure nobody is beeing nipped at.
Is there any reason why you arent moving the shrimp fry instead?
Well, I don't want to risk their survival. And ofcourse because it's quite impossible to move 50 2mm shrimplets.
What I'm thinking of doing now is to put guppies to 128l tank with lambchop rasboras, cardinal tetras, Gastromyzon loaches and brislenose. Or keep only 2 males and keep them in 5 gal til I can move them back.
But cardinals would not like higher pH and GH. And what if I get shrimplets again. Then I would have to move guppies again.
I could give guppies to my sister, but she has betta in her tank and male guppies and betta often means trouble.
Maybe I just give all away. I don't like to keep fish in that small tank anyway.