Somewhere I have heard that using multi row selection in cassandra is bad because for each row selection it runs new query, so for example if i want to fetch 1000 rows at once it would be the same as running 1000 separate queries at once, is that true?
And if it is how bad would it be to keep selecting around 50 rows each time page is loaded if say i have 1000 page views in a single minute, would it severely slow cassandra down or not?
P.S I'm using PHPCassa for my project
Yes, running a query for 1000 rows is the same as running 1000 queries (if you use the recommended RandomPartitioner). However, I wouldn't be overly concerned by this. In Cassandra, querying for a row by its key is a very common, very fast operation.
As to your second question, it's difficult to tell ahead of time. Build it and test it. Note that Cassandra does use in memory caching so if you are querying the same rows then they will cache.