If the vendor is honest, it's not too difficult to calculate.
- The computer has a "39994 mWh" battery, that is, about 40 W-h (i.e., can supply ~40 watts for 1 hour). A full charge would have to provide at least 40 W-h, and a bit more, because charging is not 100% efficient.
- The rating of battery packs seems to me a bit deceptive: they often give the capacity in mA-h, without mentioning that the cell inside is ~3.7 volts, in most cases. A 20,000 mA-h (20 A-h) 3.7 V charger could provide ~20 * 3.5 (one never lets a battery completely discharge, lest it be ruined), ~70 W-h.
- If that 20 A-h rating is honest, and allowing for inefficiency, that could fully charge your device once, with a dribble of energy left over.
However, many vendors are deceitful, misleading, and downright untrustworthy. One seller states its battery pack is 1,000,000 mA-h, because it can provide 10,000 mA-h each time it's used, if you recharge it 100 times!
Checking third-party reviews from a test site, such as Consumer Reports can help, but be prepared for disappointment.