i got 3 male guppies so far. do i have to add females because i really dont want them to reproduce but will the males get stressed and die.

They'll be perfectly fine with just males.

They won't get stressed as such, but they will probably fight a bit. The fighting might calm down after a while, but then again you may end up with a few dead fish. I probably wouldn't put anymore than 15 / 16 in a 10 gallon tank... but even that might be pushing it depending on how deep you have your gravel and how many plants / decorations you have in there.

To answer your first question no they don't need females with them. For your second question the rule according to where I work is one inch of fish per gallon. Guppies usually grow to about 2 inches. So 5 to be accurate however it will not stress them out to much to have 6. Besides if u mix a female with your boys ur tank will get very overcrowded with baby guppies.

You can put up to 20 but that is probably too deposed I have no more than 10 In my tank but most are still fry so I wouldn't put more than 10 full grown male guppies in there. Male guppies can live perfectly fine together Without a female but if you bought it and they were with females they might het depressed and if you separate the male when they have been together they will het stressed also of you do put a female inane sure you have enough because if you take any of them away from each other and they miss them they will die even I they are both males and I know this because that has happened to me