A general guideline that is used for anything that has to do with "limits", no matter if it's about the amount of words allowed or the amount of pages allowed is that you can have +/- 10%. Everything within that range is most likely acceptable, especially when it's already a considerable length. We are not talking about "one page" after all where sending 1.2 pages would look pretty weird, but 50 pages, which is already quite a lot to read.
There are exceptions of course. For example I remember taking an exam that explicitly mentioned that everything above a certain word count would simply be crossed out and completely ignored for the evaluation. If someone writes into his request that you should have at most 50 pages and that they won't read any more than that you should think about reducing the size. Either you use some tricks, like changing the font size or simply the whole font, or you reduce the margins to the top, bottom or sides. Or you need to sit down and actually try to trim your work. Or you need to sit down and think about whether you really want to do this extra amount of work instead of simply searching for someone who is willing to accept 52 pages.
But in normal writing outside of academics it would be weird to expect someone to have exactly a certain amount of pages/words/... and little variations are to be expected. Just don't go overboard with it. 52 instead of 50 is probably fine if you don't already use 8-point font size and 0.1 cm margin to all sides. You shouldn't worry about it and simply send it. If your manuscript is turned down because you submitted two pages more in a 50 page preview then you probably don't want to continue this business relationship anyway.
The worst that could happen is that someone stops reading at some point, but it's far more likely that they will stop after ten or twenty pages with another thirty to forty to go, than stopping at 50 when there are only 2 more to go.
The professional thing would be to simply send your preview and wait for their reply. The agent will understand that not every work of art can be neatly arranged into such narrow categories that you can expect someone to exactly hit the 50 page mark with your chapters.