1- When i insert more than 30 datas (at once) into a table, does sequence cache size(20) effect anything because, i use sequence for auto increment in the table.
2-What happens if i increase the cache size from 20 to 50?
The cache size is how many Oracle will cache in memory, so when you first select a next value it will read the sequence from disk (n), and write back a next value of n+20. Then as you keep getting a next value it will just give you the next value from cache until all 20 have been used, then repeat the process.
The down side is that if the server crashes or is restarted, those sequence numbers cached in memory will be lost so if you only used 10 values - there will be a gap of 10 in the sequence. This may not be a problem, but some systems require all values to be accounted for ( e.g. some invoice systems )