Way to go Teatime!
As to how many you send out, that's up to you.
All I can say in ways of advice, is what was given me, send them out in batches and wait for them to almost clear, because;
One; you might get a request for more.
Two; you might get a golden nugget of a remark from an agent with their rejection, and it always worth thinking of altering said submission in line with the comments for the next batch.
Three; you can read over the covering letter, synopsis and first three chapters just to make sure you have caught any errors.
I tend to do five snail mail ones with sample chapters and ten email query letters (normally US agents.) Then I wait for most of that batch to chear, i.e. rejections, then start again. Sad thing that I am I use an access data base for each book, tracking its submissions, and a spreadsheet for all my books/agents submitted too.
Edit: I recently had a rejection, where the agent had not used my envelope or stamps, but used another envelope with just a large letter stamp on it. I had to go to the local main sorting office, ten miles away, to collect it, and pay additional postage. What's more the rejection was not even a photocopied letter, just a thin slip of paper, bady cut! ROFL"