It's the last week of the month, which fans of Rule Breaker Investing know means they're due for a mailbag episode filled with both useful advice and humor. For this episode of the podcast, Motley Fool co-founder David Gardner has pulled a number of unusual queries from the inbox.

In this segment, he has invited senior analyst Emily Flippen and Rule Your Retirement and Total Income advisor Robert Brokamp on to help him address the question posed by Stuart, who has been unable to add to his portfolio for a while but is ready to do so again. He has one amazing performer in the bunch -- Shopify -- but too few companies overall. Should he follow the good advice that says "let your winners run, and add to them" or the equally valid advice that your portfolio should have a certain level of diversity?

To catch full episodes of all The Motley Fool's free podcasts, check out our podcast center. A full transcript follows the video.

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This video was recorded on May 29, 2019.

David Gardner: All right, and closing it out, Rule Breaker mailbag item No. 7. This is from Stuart Simpson. "Foolish friends and David, I'm finally back to a financial point where I can put cash in my investing account again!" That's a big moment for Stuart. Congratulations, Stuart! "I have a portfolio full of winners like Apple and Nintendo," and some ticker symbols I don't recognize like OHI, CGC, ACB, "oh, and one other -- SHOP." Well, that's one that we know. Shopify is a big-time Rule Breaker winner. In fact, Stuart in his note writes, "It. Is. A. WINNER!

"I know I should let my winners run wild, and this is most definitely a wild-running winner, but my Gardner-Kretzmann Continuum is like a 0.17." He's illustrating by saying, he's 34, he only has six stocks so far. "I feel like I should be adding more companies instead of adding to this winner. What would you do in this situation?"

Real quick, I'll poll my crew here. Emily Flippen, what do you think Stuart should do?

Emily Flippen: Well, you've inadvertently asked the wrong person. Working on both Rule Breakers and Marijuana Masters, in which both services Shopify is an active recommendation and a great performer, I tend to think this is a great company. Sure, the valuation looks crazy. But as you mentioned earlier, there are so many companies that, if you'd invested in when the valuation looked crazy, you were leaving so much money on the table. So, I tend to think I want to add to my winners, especially when you see a company like Shopify. I mean, they are just expanding their reach, more than any other company, even in the marijuana space, that I'm seeing right now. They've recently increased the fees for the clients on their platform and had virtually no churn as a result. That gives them so much pricing power. I see Shopify as such an amazing investment, even at today's prices -- and it's a very, very frothy valuation. I will give him that.

So, I tend to think, who cares about how many companies you own, as long as you're really sold on the ones that you do own. If you're really sold on Shopify, sure, add to it.

Gardner: All right. And, presumably for a different viewpoint, Robert Brokamp.

Robert Brokamp: [laughs] The old fuddy-duddy guy is going to give his fuddy-duddy diversification advice. Depending on what Motley Fool service we look at, we recommend that you own anywhere from 15 to 30 companies. I'm going to stick with that. He is young, he's got plenty of time to ride out the ups and downs of a concentrated portfolio. I would be much more concerned if he said he was within five years of retirement. But he should go with whatever he wants to do, but I'll stick with my standard blah, blah, blah, diversification stuff.

Gardner: With this one, I think I'm more with Robert. I mean, we all love Shopify, but with six stocks, and presumably a big-time winner that's rolling up, I definitely think you should get a seventh, eighth, ninth, tenth stock, even if the source of funds needs to be one of your big winners. If it's Shopify, which wouldn't surprise because it's up about 9 times in value over three years, that's a great source. Just sell off a little piece of it every month. Just dollar-cost average your way out of a very large position, Stuart. I think you have an opportunity to really grow a portfolio now. I like a minimum of 15 stocks in a portfolio. That's just my personal number. Others may have their own thoughts. So, at seven, you're about halfway there. A GKC of 0.17 is telling you, sir, all you need to know. Make the right call here, Stuart!