I want to use Raspberry Pi to measure temperature using LM35 sensors (15-30 Celsius degrees). I can't power up the sensor with 3,3V because it requires at least 4V and also I need about 10 sensors even if I would change my sensor Raspberry Pi doesn't have enough power on 3,3V stick. Can I connect MCP3008 with 3,3V (VDD and VREF) and my sensors with 5V? Should I use some resistors with my sensors?
You can power the sensors from 5V and also power the MCP3008 from 5V.
You need to use a voltage divider on the MISO (Master In Slave Out) line from the ADC to the Pi so that the 5V ADC digital signal becomes a Pi safe 3.3V signal.
The MOSI, SCLK, and CE (Chip Enable, also known as Slave Selects) are outputs from the Pi to the ADC and don't need any voltage conversion.