I’ve been told that having a sequel to a book will help boost the sales of the first book. Is this true? And if so, how much of an increase should I expect?
This is one of those areas that we can't exactly quantify. Sometimes a sequel boosts sales of the first book, sometimes book two doesn't even sell as much as book one. Depends.
Let's say 100 people buy your first book. (I know, you'd rather I say 1,000,000 bought your first book, but I really don't want to type all those zeroes.) They read it and like it and tell their friends. The friends plan to buy book one but never get around to it. After a while they forget about it and they never buy the book. End of story. End of sales.
Until the sequel comes along. Those same 100 people who bought book one, and liked it, all go buy book two. (This isn't reality, but pretend it is.) They read it and like it and tell their friends. Some of these friends are ones they hadn't told about book one. Some of them are the same ones they told before. Some of their new friends go buy the books, and the old friends go, "Oh, yeh. I was gonna' get book one and I forgot." So about half of the old friends go out and buy both book one AND book two. The other half plan to, but never do. And some of the new friends go buy both books too.
Then volume three comes out and the original 100 people who bought the books, plus fifty of their old friends, plus a handful of their new friends all go out and buy book three. They read it and like it and this time, they make fun of their friends who haven't read it yet—old and new. Peer pressure builds and now an additional 300 people go out and buy all three books.
So. If the first book is good, then yes, sequels help add more to their sales. If the books aren't so good, you'll see everything in the reverse, with fewer people purchasing each new book in the series, until eventually the publisher decides it's not worth his/her time and money.
But that would never happen to YOU because YOUR book is going to be wonderful, right?