I would like to sing while playing the guitar, but do not know how to keep with the rhythm. How many strums are needed before someone must sing? I am confused, how i can try it?
There isn't a set "number of strums", really. Also, I tend to agree with Johnny Strings to start simple.
One thing that may also be helpful is to simply make sure you hit one, single chord, while at the same time singing the first word of the verse. Let the chord ring and continue singing through the verse until the next chord comes up. Do the same thing with the next chord. Keep working your way through the song, hitting the chord only once when it changes. Don't worry about doing anything too complex on your guitar.
Once you do this a bit, you'll probably be able to get your singing to be rhythmic and in tempo and you'll have a "feel" on when to strum the chord changes. After that, it get a bit more natural where you don't need to spend so much mental effort on either playing or singing, and then you can work out a bit more complex pattern
I would say when working on anything where there's more than one thing happening at the same time, whether it's playing an instrument and singing or something like drums where different limbs can be doing quite different rhythms, the following might help:
Do it at the absolute slowest speed first. I mean make it EXTREMELY slow so that you have time to think about what's happening and what you're going to do next. Don't worry about doing it at tempo. Break it down as much as you can into smaller pieces and do them correctly but super, super slowly. It will probably seem like a pain at first, but trying to do it more quickly than you're ready for can be too big of a step sometimes. In addition, you could learn something incorrectly and turn it into a habit
Focus on the order of things, including what things happen at precisely the same the same time. Write it out on paper if you have to. If the pattern is
[strum] [strum] [strum] + [sing first word at the same time] [sing 2nd word] [sing 3rd word] [strum] + [sing fourth word at the same time]
writing it out like that makes it easier, as you can just read what you need to do as you do it slowly and develop muscle memory.
If you can give us an example song you're working on, perhaps we can help you break it down.
The hard part is when either vocal or guitar are in different rhythms. I would start with songs where the guitar and vocal are rhythmically identical. So with each strum you sing a new word. From there proceed to only slightly more complex vocals.
And for practice you don't even need words... just start strumming anything slowly, and then sing "La" with each strum... once you're confident, move to "La-la" then "La-la-la" and eventually to combinations of all of these... and finally to a point where the vocal barely even follows the guitar other than syncing up the chord progression with the beginning of each new lyrical phrase.
Also, google it: "how to play and sing at the same time." There are lots of thoughts out there on it... this is just what helped for me.