How does a composer decide how many violins, cellos, etc., to use for his composition?

The composer usually does not recruit people for a concert.  School bands adapt depending on who is available.

It is up to them. They may want to use a full-size symphony orchestra, which typically uses 10 each of violas, first and second violins, 8 cellos and 6 double basses. Or they might want to use a chamber orchestra (typically 4 firsts, 3 seconds, 2 violas, 2 cellos, 1 double bass).

That does not reflect how many individual parts there are - a larger orchestra doesn't necessarily have 10 times the number of musical lines in the string section, it is just louder. String instruments are not really very loud - in the days before amplification the only way to get comparatively quiet strings to be heard over much louder instruments like the trombone, piccolo and oboe was to have more of them - a LOT more, as the increase in sound isn't linear - it's logarhythmic.

As for how the composer gets hold of their musicians, they typically work with an established ensemble - often being comissioned to write a work specifically for them that is later licensed to be published.

no he do not