How to find the number of CPU cores including virtual?

You can count no of CPUs

cat /proc/cpuinfo | grep processor | wc -l

Output :

2

To check the number of cores !

cat /proc/cpuinfo | grep 'core id'
core id         : 0
core id         : 1

Or

 $ nproc
 2

Or lscpu will show you all output:

lscpu

Architecture:          i686
CPU op-mode(s):        32-bit, 64-bit
Byte Order:            Little Endian
CPU(s):                2
On-line CPU(s) list:   0,1
Thread(s) per core:    1
Core(s) per socket:    2
Socket(s):             1
Vendor ID:             GenuineIntel
CPU family:            15
Model:                 4
Stepping:              7
CPU MHz:               2792.992
BogoMIPS:              5585.98
L1d cache:             16K
L2 cache:              1024K

To add to the existing answers, you can determine information about Intel's HyperThreading by looking at the "siblings" line in /proc/cpuinfo. The example below is from a 2 socket machine. It shows the CPU has 6 cores but 12 "siblings". On Intel CPUs this means HyperThreading is enabled and there are 6 physical cores.

processor       : 23
vendor_id       : GenuineIntel
cpu family      : 6
model           : 62
model name      : Intel(R) Xeon(R) CPU E5-2430 v2 @ 2.50GHz
stepping        : 4
microcode       : 0x428
cpu MHz         : 1599.707
cache size      : 15360 KB
physical id     : 1
siblings        : 12
core id         : 5
cpu cores       : 6
apicid          : 43
initial apicid  : 43
fpu             : yes
fpu_exception   : yes
cpuid level     : 13
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc aperfmperf eagerfpu pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm ida arat xsaveopt pln pts dtherm tpr_shadow vnmi flexpriority ept vpid fsgsbase smep erms
bogomips        : 5005.20
clflush size    : 64
cache_alignment : 64
address sizes   : 46 bits physical, 48 bits virtual
power management:

dmidecode is also useful for determining what hardware a Linux system is running on.

/proc/cpuinfo contains all the CPUs for my computer, including virtual. You can count them with a little grep:

grep -Pc '^processor\t' /proc/cpuinfo