Generally, usb can provide up to 500mA. The Wemos consumes around 80mA on average, when connected to WiFi. There are spikes of much higher current use but these are very short, so if you attach a large cap to the 5V pin, like 470uF or even 1000uF, that should take care of those. This leaves your circuit with about 420mA in theory, but you should plan to stay well clear of that in practice.
7 x ws2812b could in theory consume 7 x 60mA = 420mA. That's right on the limit, but in practice these LEDs rarely consume that much. All 7 LEDs at full brightness white would be the maximum current. Full brightness in any single colour like red/blue/green would only be 1/3rd of that.
The two main libraries for controlling ws2812 (NeoPixel and FastLED) both have an overall brightness setting which will help you control how much current will be used. The FastLED library even has a setting which will allow you to specify the maximum current/power and automatically reduce the brightness of the strip to stay under that max.
So I think you will be ok with 7 LEDs. It would be interesting to hear back from you about how much current your 7 LEDs are consuming on full brightness white (using a multimeter).