Two directions: how much stock you need to buy to hit a monthly payout, and what the shares you already own pay today.
Annual need = monthly income times 12. Portfolio required = annual need divided by the yield as a decimal — the definition of yield rearranged, since yield is income divided by price. Shares needed = portfolio divided by the share price, rounded up, because you cannot buy part of a share. Annual income per share = price times the yield as a decimal. Reverse mode runs the same identity forwards: shares times income per share is the annual figure, and one twelfth of it is monthly.
A $2,000 monthly target is $24,000.00 a year. At a 3.50% yield that needs a portfolio of $685,714.29. With the share price at $50.00 it is 13,714.29 shares, so 13,715 whole shares, which actually cost $685,750.00. Each share pays $1.75 over a year ($50.00 times 3.50%), or $0.44 a quarter on the usual US quarterly cycle. In reverse, the 100 shares in the default box pay $175.00 a year and $14.58 a month — 0.73% of the $24,000.00 target, so 13,615 more shares to buy. Same target at a lower yield, and the number runs away: $1,200,000.00 of stock at 2%, $400,000.00 at 6%.
A yield is not a promise. A board can cut or suspend a dividend at any time, and the eye-catching yields are usually first to be cut, which turns the share count above into a much larger number. A yield quoted from the last twelve months of payouts is not the yield you will receive: it divides dividends already declared by a price that has moved since. Price risk is outside the model — income can arrive while the capital producing it falls. A dividend is a transfer from the share price to your cash, not a return in itself, and taxes come out too.
Arithmetic only. Nothing here guarantees an income, a payout or a price. Check any target against your own tax position and, where relevant, a qualified adviser.