@Pesi S. There are some who will say you should only ever pay cash, and others that will say you should always leverage. It really depends on your comfort level, specific situation, and other factors important to you. The benefits to having leverage include tax avoidance, liability protection (tenants tend not to sue leveraged properties since there'd be much less to recover), great ROI due to lower cash invested, etc. I'd say a good ratio is anywhere from 75%/25% to 60%/40% for leverage to equity. Most banks tend not to refi for anything more than 75% of the market value.
The key is to make your money going into the deal. If you've done the math, are conservative in your estimates, and still make positive cashflow after paying all expenses and liabilities, you should be able to weather market conditions. Most people got burned in the market crash, me included, because they were gambling with market appreciation. People were buying houses despite the negative cashflow betting that they'd sell eventually and make a ton of money on the appreciation regardless of the short term cash loss. That turned out badly for many people. Smart investors were getting out early before the crash, or weathered the storm because they did their math right.
As for how many loans, a conventional bank will limit you to between 3-10, with most stopping well before 10 loans. Most folks look for seller financing, portfolio lenders, or private investors at that point. The key is not to overextend yourself and have a good exit plan.
I'd highly recommend Brandon Turner's "The Book on Rental Property Investing". You can buy it on BP and download it in pdf, hard copy, or audio. It covers everything we've just discussed and a ton more.
Jim