How can I know the number of open TCP streams in the pcap file?

If you just want to know how many TCP streams are in a pcap file you can use the Conversations Statistics and look at the number given on the TCP tab.

If you then want to find out how many TCP streams are still open at the end of the trace file you need to find out how many of the total TCP streams have terminated within the trace file. Filter for tcp.flags.reset==1 or tcp.flags.fin==1 and open the conversation statistics again, select the "Limit to display filter" option at the bottom and take a look at how many TCP conversations are left. This the number of streams that have been terminated either with a reset packet or a fin packet. Substract that number from your total stream count and you should be done.