As new processes fork in, PIDs will increase to a system-dependent limit and then wrap around. The kernel will not reuse a PID before this wrap-around happens.
The limit (maximum number of pids) is /proc/sys/kernel/pid_max. The manual says:
/proc/sys/kernel/pid_max (since Linux 2.5.34)
This file specifies the value at which PIDs wrap around (i.e., the
value in this file is one greater than the maximum PID). The default
value for this file, 32768, results in the same range of PIDs as on
earlier kernels
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