Bookie money can be withdrawn whenever you want. Exchange money also, UNLESS it's tied up in liabilities, in which case you either need to wait for your lay bets to settle, or you need to cash out your lay bets.
Free bets are usually restricted in stake to £10 or £20 so that is the maximum stake usually. sometimes they're more though. However yes if you had loads of spare cash you can of course do multiple £10 free bets with lots of bookies, but again you'd need the money n the exchange to cover liabilities.
People often spend the first few weeks not withdrawing, just building up the money they have in their exchange account. A good spreadsheet keeps you motivated though!
Finally, if you have several free bets, you can put them on different outcomes of the same event (e.g. On Man C vs Man U you might put a free £10 from William Hill on Man U, a free £10 at Ladbrokes on Man C and a free £10 at Coral on the draw), then you only have one liability to cover at the exchange, because clearly only one of those outcomes will happen.